Every so often, a hero (usually a police officer or spy) will pretend to go to The Dark Side in order to reach the Big Bad. This is not the same as The Infiltration; in this case, the hero keeps his true identity, but convinces the bad guys that his loyalties have switched. Frequently many of the good guys end up believing it too.
The hero will often have to persuade the bad guys of his authenticity by attacking one of his 'old' comrades or doing something else evil. Another frequent tactic is for the hero not to contact the bad guys, but instead just arrange for his life to fall apart — get fired from his job, start to drink, and so on — and wait for his enemies to contact him.
The audience is usually told in advance that the hero is doing this, but occasionally they are not. Often it is subject to a sort of Unspoken Plan Guarantee: if the audience knows, the bad guys are more likely to know as well.
Compare the inverse, the Heel Face Mole.
As a betrayal trope many examples will contain spoilers.
This scene often appears in the Sera Myu, or stage musicals, and even has its own song.
In an earlier arc, Sailor Moon herself feigned a falling out with the other senshi and attempted to join the Dark Kingdom so she could find the entrance to the Dark Kingdom. Pity she tried this on the most cunning of the Dark Kingdom's servants...
Digimon Savers: Kurata rolls out the red carpet Touma to do this. Touma goes so far as to get into a fistfight with Masaru and nearly kills Ikuto and Yatagaramon.
Nancy does this with Yomiko in the Read or DieOAV, while quoting "to deceive your enemy, first you must deceive your friend".
Mirage pulls one of these in Transformers: Robots In Disguise. After having a genuine fight with his team, he realizes that the Predacons have been listening in and promptly takes advantage of the situation to pretend that he wants to defect.
Chie pulls this off in Mai-Otome by pretending to side with the Valkyries as part of two separate plans:
To wrest them and any other Otomes who may have sided with Nagi from his control, and
The event reshapes the entire series and leads to huge amounts of character development—you can pretty much divide the series into 'before' and 'after' this arc, with a brief transition period between his disappearance and his reemergence as Belial's new Dragon.
The audience is not let in on the arrangement, nor is any other living hero. We are given blatant foreshadowing of the apparent Face-Heel Turn about three minutes before The Reveal.
Conrad wears a similar coat and has a similar scene in the same arena in season three, and you can see Yuuri's stomach turn over as he says half-jokingly 'not this again.' Conrad reassures him. Then he declares his own bid for the Shimaron throne on the strength of an unbroken family line that lost power four hundred years ago. Certainly messes with everyone's plans.
The fact that they still have Conrad's severed left arm in its original sleeve while he's walking around with two working arms is a big tip-off that something more convoluted happened than just the most loyal person ever deciding to betray his king, or even Brainwashed and Crazy. It's basically 'cause God Is Evil. Or..possessed by evil. Basically God is playing Xanatos Speed Chess with himself, without anyone else suspecting there's more than one will nudging the pieces. Yuuri is the culmination of the good and evil plots, because there's no way either of them would have succeeded in getting there if they hadn't built on the other's foundations for their own purposes. You Can't Fight Fate and yet he very much has a choice and control.
In Naruto, Itachi Uchiha of the criminal organization Akatsuki is revealed after his death to have been a double agent who was loyal to Konoha the entire time. In fact, the real reason he killed his entire clan was to prevent a civil war which would have devastated the village; in exchange, his superiors promised that they would spare his brother Sasuke.
Viewing his previous behavior in light of this revelation makes it pretty clear that he was doing best to delay Akatsuki and not kill anyone.
In the X1999 anime, Arashi joins the Dragons of Earth to save her boyfriend Sorata from sacrificing himself for her. When Fuuma gives her an order to kill him though, she betrays him. Sorata almost manages to take Fuuma down... but he still ends up dying for her sake.
In Dragon Ball GT, Gill appears to betray the team to the machine mutants. Only Trunks is in on the plan.
In the Garlic Jr. Saga of Dragon Ball Z, Piccolo did a variation of faking defection to Garlic Jr: He ended up being bitten by one of the Black Water Mist-infected characters, and then faked infection himself and fought Gohan and Krillin ruthlessly in order to get close enough to Garlic Jr. and then free Kami and Mr. Popo. He only let Krillin in on the plan when he was "biting" him on the neck, resulting in Krillin faking defection as well.
In the The Legend of Zelda: Four Swords Plusmanga, the Violet Link pulled this off in order to learn of the enemy's plans, even knocking out the Green Link in order to prove his "loyalty".
Lelouch, along with Suzaku from Code Geass pull this off at the end of the series. They pretend to become the new villains who take over the world, but it turns out Lelouch had planned his own death to bring world peace.
Byakuya does a pretty good one in the Zanpakuto arc. The ally he has to kill is his sister Rukia's Zanpakuto, Sodeno Shirayuki, and he does it without flinching (of course).
Uryu Ishida pretends to join the Vandenreich in an attempt to assassinate their leader, Yhwach. However, Yhwach sees through the ploy immediately and appoints him his successor, meaning everybody's eyes will be on him and an assassination attempt will be all but impossible.
It is implied in the beginning of the Manaphy movie that Jack Walker had to pull off a fake defection in order to get close to Phantom and retrieve Manaphy's egg to prevent him from accessing the Samiyan Sea Temple.
Hinata from Bloody Cross. At first it looks like he's a traitor who tried to kill Tsukimiya and sided with Arcana, but it turns out he's a fake defector who's still working for Tsuduki.
In One Piece, in the filler Alternate UniverseDetective Memoirs of Chief Straw Hat Luffy, Zoro, a travelling monk, appears to be an Aloof Ally, helping out Vivi when the Buggy Clowns chased her. Buggy decides to hire him to defeat Luffy. But during the fight, Zoro intentionally has Luffy hit all of Buggy's crew. And when Mohji and Cabaji tries to attack Vivi, he stopped them.
Recently in the story proper Brook pretends to enjoy Jora's "art" in order to trick her into changing his cane and violin back to normal so he can cut her up.
In the Hades saga of Saint Seiya, all of the Gold Saints who died during the events of the 12 Temples arc are ressurected after swearing loyalty to Hades, God of the Underworld. Thus, they lead an invasion to the Sanctuary with the intent of assassinating Athena, eventually succeeding it. We then learn that the reason behind their betrayal was to a)kill Athena so she can awaken her Eighth Sense and descend into Hades's Underworld without being bound by its rules, and b)shed the Goddess's blood in order to ressurect Athena's Cloth so she can use it to fight the Lord of the Underworld.
In BeelzebubHimekawa pretends to desert his True Companions in favor of a stronger guy. Turns out the machine everyone thought was helping his new crew out was actually storing up Baby Beel's energy until it was enough to handle Takamiya and Lucifer. Himekawa proceeds to destroy the million-dollar machine to release the energy the second he has confirmation that it is enough and dismisses the loss as "pocket change" before revealing he held Beel's third crest all along, all but stating that his loyalty has always been to Oga.
Chiyuri from Accel World ended up in this for joining Dusk Taker, but it turns out she didn't. She only sticks on his side to become strong enough and see that her ability known as 'Citron Call' can reverse time to give back Haru's wings that Nomi has stolen from him.
Nightwing becomes a fake defector in the year following the Infinite Crisis storyline. This was done to give him additional credibility among several rival gangs. His success is debatable; on one hand Slade asks him to train his daughter Rose as the new Ravager, but on the other hand, his actions lead to an encounter with Superman. Revealing both Slade's insight and foresight, Slade prepares a gizmo to give Nightwing's heartbeat the sound of a truthful man rather than a liar.
Post Badass Decay, Juggernaut (an X-Man at the time) is ordered to fake-defect to gain intel on the New Bastards on the Block. Unfortunately, Juggernaut's kindred-spirit Squid Boy was not in the loop and, getting caught in the crossfire during a battle, tells Juggernaut he'll never forgive him before promptly expiring. Ouch.
The Trickster and the Pied Piper attempted to infiltrate their old friends, The Flash's Rogues Gallery — an apparent reversal of their Heel Face Turns. When they appeared to have been complicit in the murder of Bart Allen, the Flash, they found it impossible to convince people of their sincerity.
Star Wars: Dark Empire had Luke Skywalker falling to the Dark Side, if only to try to bring it down from within.
The Squadron Supreme limited series sees the entire team pretending to be brainwashed into joining the Institute of Evil.
The first story arc of Ultimate X-Men has Cyclops storming off and joining Magneto's Brotherhood of Mutants. Guess why he does it.
In The Chong Sheng Trilogy's third installment, Mai does this in a particularly horrifying example. Subverted slightly in that she's also trying to avoid execution with the rest of Zuko's advisors. In order to prove it, she's forced to torture a captured Zuko while Katara is Forced to Watch. Zuko buys the act completely at first, but Mai lets him know her true intentions by the marks she carves into his arms.
In Winter War, Hisagi apparently defected to Aizen during the Fake Karakura Town battle when it became obvious that he was going to win. He did so at Yamamoto's orders... but with Yamamoto and everyone else who was in on it dead, La Résistance believes he defected for real. Even when he contacts them and offers to help them invade Hueco Mundo, they have no way of knowing if he's genuinely on their side... and neither does the reader, until we get a chapter in his POV a while later.
Zecora in Fallout: Equestria. Tragically, her plan to assassinate the Zebra Caesar might have worked if she hadn't ran into Applesnack during her 'escape'. He beat her to death.
There's an example of a reverse of this trope in The Living Daylights in which Koskov pretends to defect to the West. Turns out he's playing both sides against the middle and used the defection as an opportunity to get his girlfriend offed by James Bond (who doesn't actually do it and ends up taking off her clothes later in the movie).
Kyra joining the Necromongers in The Chronicles of Riddick, only to help Riddick when he faces the Lord Marshal in the final fight.
In X-Men: First Class, Darwin does this to Sebastian Shaw's group of evil mutants as a ploy to give his teammate Havok a clear shot at Shaw's group (Darwin's own mutant ability will protect him from Havok's attack, and their friend but genuine traitor Angel, who wouldn't be safe otherwise). It doesn't turn out so good for Darwin when Shaw's group survives unscathed due to Shaw's mutant ability allowing him to absorb Havok's attack.
Interestingly done in The Prestige. Angier sends his assistant, Olivia to work for Borden and feed him information. Knowing Borden would be suspicious, he tells her to admit that Angier sent her as a spy, but to claim that she was so mad at Angier for treating her like a pawn that she's willing to feed him information about Angier. In other words, the whole plan is completely transparent, both sides know full well she's working as a double agent, but neither can be sure whether she's a defector or a fake defector.
In The Godfather, Don Corleone send Luca Brasi to infiltrate the gang of Sollozzo to garner information, but Sollozzo is wise to the plot, and Luca ends up sleeping with the fishes.
The trope is used as a minor Mind Screw to the audience and a major one to the protagonist in Total Recall (1990), as Hauser tricks Quaid who is Hauser, post memory wipe into thinking he has betrayed Cohagen when in fact it's all an elaborate gambit by Cohagen. Throw in not one, but two moles and you've got one hell of a manipulative conspiracy.
In the charmingly goofy family B Movie Mystery Monsters! (Released on DVD and Netflix as Goobers!), protagonist Tommy's plan for freeing the monsters involves convincing Big Bad Queen Mara that Jerkass Jimmy has sold Tommy and Susie out. The audience is not informed in advance, and to try to sell it that he's really betrayed them, Jimmy's Heel-Face Turn beforehand is seemingly transparently phony, with a little more than a Heel Realization that he's a selfish, obnoxious jerk who is both hated by everyone and deserves to be spurring on his desire to be something else.
In I, Robot, Sonny pretends to be swayed by VIKI's "undeniable" logic, and suddenly holds Dr. Calvin hostage. It's all a ploy to get them safely out of the room with VIKI so they can head towards her positronic brain and shut it down, but it's only when Sonny winks at Spooner (a gesture Spooner had earlier described to him as a sign of trust) that he and the audience realize what's going on. Poor Dr. Calvin is still out of the loop.
Dr.Calvin: (as they rush up the staircase) I assume we'll discuss what just happened later?
During the climax of The Negotiator, Roman is trying to convince Big Bad Frost to talk, but Sabian notices that it's not working, drags Roman out of hiding, and shoots him, stating that in compensation, he wants half of the money that Frost stole. Frost agrees, destroys the evidence, and walks out of the house to find that his entire agreement with Sabian was broadcast to the police, implicating him quite effectively. Sabian only shot Roman non-fatally.
In Counterfeit Traitor, Eric Erickson is this, in the movie dramatization of what he did, as noted in the "Real Life" section below.
In Star Trek: First Contact, Data is seduced by the Borg Queen with the promise of becoming more human by having human skin grafted onto his body and serving as her consort. He pretends to join them by deactivating the Enterprise's self-destruct and firing at the Phoenix, before killing the Borg Queen and defeating the Borg single-handedly. Data notes that he actually considered the offer for 0.68 seconds — for an android like him that is an eternity.
In A Song of Ice and Fire, the beyondhonorable Jon Snow becomes a fake defector in order to spy on the wildling king Mance Rayder; he is forced to kill Qhorin Halfhand in order to gain their trust. And then to have sex with his new wildling girlfriend every night to maintain their trust. I mean, how else to convince them he's forsworn those vows of chastity he loves so much?
Hey, to be fair he does feel pretty bad about it! ... for about a chapter...
The Westerling family counts as well. The family is of minor nobility and are sworn to the Lannisters. After their daughter Jeyne, likely the only family member who doesn't fit this trope, marries King Robb Stark, leader of one of the armies opposed to the Lannisters in the War of Five Kings, they pretend to declare loyalty to the Starks. However, Robb's direwolf Grey Wind, an Evil-Detecting Dog, indicates that something is wrong about the family. Later, Jeyne's mother reveals to Jaime Lanniester that she had been in contact with his father, the head of the Lannister family, and was aware of the planned betrayal of Robb Stark at the hands of one of his sworn families. She also gave her daughter drugs to prevent conception, thus ensuring the male line of the Stark family died with Robb (or so she thought).
Wyman Manderly, ever loyal to the Starks, who pretends to defer to the Freys and Boltons when it's really all a ruse to dish up some revenge of his own.
John le Carré novels use this a lot - with large amounts of detail on how you go about doing this (slowly dropping out of society, becoming a drunk, etc).
Successfully pulled by Sir Horace Harkness in the Honor Harrington series, when their ship was taken by Havenite State Sec. A life of smuggling and bootlegging certainly paid off — he managed to deceive his watchdogs, hack the enemy ship's central computer, free his mates, and blow the entire ship to Kingdom Come after making a clean getaway. He was deservingly knighted for this performance.
Professor Severus Snape of the Harry Potter series. Interestingly, he defected to both sides at different points, so we're kept guessing for most of the series about which side he's a fake defector to. (Fans debated it to hell and back.) It all ends with a very unusual twist: his deception is successful but his mission fails. Voldemort believes Snape, but kills him anyway, out of expediency. That said, he was able to prevent a whole lot of worse abuse towards the students of Hogwarts using his position as Headmaster, and was able to give Harry his all-important memories about Voldemort's extra Horcrux, so in a way he succeeded.
Subverted in Emperor: The Gods of War by Conn Iggulden, where Brutus actually has done a face-heel turn and betrayed Caesar. However Caesar and his generals play it out as a fake defector, and even Brutus himself does so in order to get into Caesar's daughter's bed.
In Watership Down Bigwig tells the Efrafans that he's a member of the Owsla who escaped the destruction of his warren by Men, which is true except Bigwig neglects to mention that he's part of a group of similar refugees hoping to spring some of the Efrafan does for their own warren.
Lara of the X-Wing series started off as a bad guy playing The Mole in the New Republic. She caused the near-total destruction of an X-wing squadron, then went back to the Imperials, got disgusted by her newest boss's insistence that We Have Reserves, and engineered his death. Then, awaiting another chance, she went undercover among the New Republic again, joined an X-wing squadron, and started Becoming the Mask and finding that Good Feels Good, so she switched sides and started getting close with the only survivor of the X-wing squadron she'd destroyed. Then her true origin was revealed and she was forced to flee and rejoin the bad guys as a Fake Defector, contributing to a major New Republic victory. It ends on a bittersweet note at first (she has to live the rest of her life under a different identity to avoid being condemned to death for her actions as an Imperial agent), but a later book shows that things went all the better for her, as she started both a family and a successful business with the aforementioned survivor.
In Orson Scott Card's novel Shadow Puppets, Suriyawong spends most of his time working his way up to being the Big Bad's second-hand man so that when the time comes he can give the good guys a chance to shoot the Big Bad in the head.
In Tim Powers's novel Declare his protagonist Andrew Hale is more or less shoved into making a false defection in order to get close to his nemesis Kim Philby.
Happens many times in Romance of the Three Kingdoms. Especially noteworthy is just before the battle of Chi Bi; after Cao Cao's loyal admiral is executed thanks to a plot of Zhou Yu's, the admiral's brothers use it as a cover story for their supposed defection. However, they are easily discovered, but Zhou Yu just proceeds to play along so as to plant his own Fake Defector to defeat Cao's giant navy.
Also common in Kamakura-era retellings of the Tale of Genji. Many scenes in Genji make it clear that Genji was having an affair with the Emperor's chief wife, and even fathered her child. To avoid the unpatriotic implications, the retellings would have them go through the same conversations... but later reveal they were faking the whole thing to fool their enemies! And then NINJAS ATTACK!
As of The Gathering Storm, Verin is revealed to be one of these, having joined the Black Ajah to save herself, then studied it and eventually brought it down from within. Considering how well she fooled her fellow Blacks, and the Loophole Abuse she used to be able to divulge all (which also entailed a Heroic Sacrifice),it's tempting to call her a Magnificent Bitch despite still being one of the Heroes. She's certainly mistress of The Plan.
Briefly done by Ax, of all people Andalites, in Animorphs. It was a spur-of-the-moment idea, and he only stuck with it long enough to get into a better position to fight back.
In a more extended version, Ax fakes defecting to, er, his own side, technically—when a small Andalite squad comes to Earth, the Animorphs (rightly) suspect their motives and pretend to break up, freeing Ax to go join up with the Andalites and find out what they're up to.
In Tom Clancy's Cardinal of the Kremlin, Jack Ryan pulls this, staging several 'incidents' to trick the Russians into trying to recruit him, including a rather public incident involving a gay senator. He's not actually going undercover, though — he just needs to arrange a one-on-one with the KGB Chief, so he can blackmail HIM into defecting, bringing along the titular mole.
In the Star TrekDouble Helix novel "Double Or Nothing", Mackenzie Calhoun needs to infiltrate the Big Bad, so he punches out an admiral. (He was supposed to have a loud argument with him, but he decided that he needed to be punched...to make the charade more authentic, of course). It is subverted, however, in that the Big Bad sees right through it and keeps it hidden until the big moment.
The female lead of the James Bond novel and film From Russia with Love was told that her mission was to become one of these to leak false intelligence to the West. Her mission is actually a set up to lure Bond into a situation where SMERSH/SPECTRE (Depending on the adaptation) can kill both of them in a manner that embarrasses the British government.
In The Dresden Files novel Changes, Martin pretends to defect to the Red Court, serving as a Double Agent and setting up their total destruction. The protagonist was incredibly surprised, naturally.
In Lucky Starr and the Rings of Saturn, Lucky Starr allows himself to be captured by Earth's enemies, the Sirians, and then gives away one of his fellow Councilmen of Science to the Sirians and testifies about his betrayal for Sirius at an interstellar conference. Of course, it's all an elaborate ploy to get on the stand and present testimony that is devastating to Sirius. The act is especially hard on Lucky's sidekick and best friend Bigman, who believes Lucky is turning traitor in exchange for his, Bigman's, life.
Lucky: [You] gave me the opportunity to make it look as though I were sincerely swapping Wess's freedom for your life. It took less acting to do that than to give Wess away under any conditions I could have dreamed up in your absence. In fact, as it was, I didn't have to act at all. It was a good swap.
In The Hunger Games it seems like Peeta has teamed up with the careers to kill Katniss. In actuality he teamed up with them to lure them away from her and help keep her alive.
In Christian Nation, the protagonist convincingly fakes being "born again" so that he could escape being executed by the American theocratic government.
In Black Company you have Blade, who "defects" after Croaker suspects him of having an affair with Lady, Croaker's wife. In the months long con that followed, Croaker repeatedly sent the religious fanatics of his own army against Blade's forces, thinning his pressed army of the malcontents while Blade turned nearly his entire enemy force to the Company side.
Live Action TV
Wolf of The 10th Kingdom is a particularly well-done version of this trope. It keeps you guessing, even right up to the very end, which side he will truly turn out to be on. It helps that, as a deconstruction of many fairy tale tropes, Happy Ever After was not guaranteed by any means, and the ending still remains rather dark...
In another episode of The Next Generation (appropriately titled "The Defector"), a Romulan officer falsely claims to be a middle-ranking clerk of negligible importance who had come across some important information about a secret base which the Romulans planned to launch an attack on the Federation from. His defection was sincere, but he was lying about his identity. He wanted to warn the Federation about the attack so a war could be prevented, but he did not want to further betray his homeworld. He finally admits that he is an Admiral and agrees to hand over any required information to the Federation. Tragically, it turns out that he is the Unwitting Pawn, fed false information by Commander Tomalak to lure the Enterprise into crossing into the Neutral Zone first so they could ambush them and justify an invasion of the Federation. Fortunately, Captain Picard arranged ahead of time for the Klingons to sneak in with them in case they needed rescuing. Both sides retreat without a war starting.
Star Trek: Voyager has a subtle variation. Tom Paris starts showing up late, picking a fight with his superior, starts the crew gambling over multiple episodes. He lines up a new job and is leaving the ship forever. Instead of getting approached by the other side they decide to kidnap him, exactly according to plan.
Jack Bauer in 24 series 3, breaking out Ramon Salazar from prison was an elaborate sting operation. In order to convince the Salazars that he had gone to the bad side, he had to point an (unloaded) pistol at Chase's head and pull the trigger. He is assisted by Gael Ortega, who goes so deep undercover as a mole that CTU arrests and interrogates him before realizing their mistake.
Done again by both Tony Almeda and Jack in the season 7. Also Greg Seaton fakes a bad-to-good defection.
Jack Bristow in Alias, pretending to leave the CIA to work for SD-6.
Gary Best in The Bill, whose undercover assignment involved faking his sacking for assault. Adam Okaro and Lewis Hardy also did this. The latter shot to miss Roger Valentine.
Mission: Impossible had at least one such episode, if not more. And then there are the episodes where they have to convince an enemy agent that a real defector is actually a fake defector so that the intelligence they leaked will be disregarded.
Angel used a much shorter version of this in Spike's first episode. During Spike's attack on Sunnydale High, he strolled right up with Xander in a headlock, claiming to have been playing Buffy. Spike didn't buy it.
In several episodes of the second season of Star Trek: Voyager, Tom Paris becomes sullen and insubordinate so his fake defection (part of a mission to root out an informant) looks real.
He goes as far as setting up a gambling ring and skimming from the betting pool to increase the appearance of an unreliable, unethical officer.
Battlestar Galactica: In order to get to her daughter on board a Cylon Baseship, Athena tells Caprica Six that she thinks that Hera is safer there.
Boomer does that too. Ellen thinks that Boomer helped them escape. Boomer only pretended so to get access to Galactica and steal Hera.
Stargate SG-1: In the episode "Shades of Grey", Colonel O'Neill gets suspended from duty for stealing technology from aliens. This allows him to infiltrate a group of bad guys, but in the process he alienates his team, who are quite shocked at his behaviour (not being in on the plan). Of course, they're all friends again by the next episode.
Interesting thing is that in that episode Daniel appears to have a friendly talk to Jack at the end, and reveals he came because he lost the draw.
Teal'c fools Ba'al into believing he was brainwashed in "Stronghold".
Teal'c is also brainwashed by Apophis into thinking that he was this trope, when he was "really" still loyal to Apophis.
Carter also had to pretend to be infected by a Goa'uld symbiote clone among an entire town infected by them to trick them into being arrested by the SGC. In order to sell the deception, she also had to hit one of her allies beforehand.
The opening quote of this article, from Star Trek: Deep Space Nine, turned out to be in a holodeck scenario. But the scene was such a perfect fit to Garak's devious personality that most fans forget he didn't really do it.
In Crime Story, the lawyer for the Federal task force that is trying to take down Big Bad Ray Luca, pretends to be drunk and gets kicked out of the agency, so that he can become Luca's lawyer, and then inform the task force about everything Luca is doing. However, Luca points out to one of his henchmen who questions whether the lawyer would be loyal to Luca, that even if this lawyer was pretending to go to his side, because of lawyer-client confidentiality rules, nothing he tells the feds can be used against him.
In the LOST episode "The Economist," we see Hurley questioning Locke's leadership. In his next scene, he tells Sayid and Kate that he has abandoned Locke's group. It turns out to be a ruse, to which the audience was kept in the dark.
Also, Benjamin Linus pulls this in "What They Died For", even going as far as to kill Charles Widmore (though that part probably wasn't an act) to convince the Man in Black where his loyalties lie. Oddly enough, he doesn't actually accomplish much, other than to allow Miles and Richard to escape the Man in Black's wrath and eventually get off the island.
Sgt. Miller pretended to collaborate with a criminal gang in an episode of Heartbeat, though the writers didn't even try to explain why he didn't tell the other police what was going on.
Hiro from Heroes pretends to stab Ando with a katana in order to join the other side.
In the Pushing Daisies episode "The Norwegians", Olive resents the fact that she is perpetually Locked Out of the Loop and decides to join the Norwegian detective team that is trying to put Emerson out of business and would have exposed Ned in the process. It's revealed that she was faking all along in order to throw them off the track.
In the Sentai show Kousoku Sentai Turboranger Pink Turbo winds up joining the bad guys in attempt to recover an antidote for the poisoned Blue Turbo by fighting alongside the monster of the week and "accidentally" getting poisoned by it as well forcing it to give her the antidote. Oddly enough this didn't seem to be planned in advance at all, she just happened to take a hit to her head during a battle and decided to fake memory loss to join the bad guys. The other guys had no idea what she was doing until the Reveal when Blue was shown to be healed. She even was perfectly willing to shoot a machine gun at them as part of her plan.
Chouriki Sentai Ohranger repeated this plot only this time had it fail. Ohyellow joins the bad guys in attempt to recover an antidote however unlike Pink Turbo faking amnesia Ohyellow faked cowardice and surrendered instead. Ohyellow also lets herself get hit by the poison darts and tries to get the antidote however she wore a bulletproof vest to protect herself unlike Pink Turbo who allowed herself to be poisoned. Since Ohyellow doesn't take as many risks the bad guys aren't fooled and give her a fake antidote instead. They then need to use a backup plan instead.
A not-so-much copy-paste-from-Turborangers Fake Defector plot is also shown in Choujin Sentai Jetman, the 'fake defector' none other than Red Hawk himself, pretending to join the bad guys so they will return his girlfriend-turned-evil. Turns out he was trying to sneak into their base and destroy their mean to transport themselves into the real world. Plan failed, but in the end, their chief praised him and his teammates for being powerful and cunning enough to break through the enemy's defenses.
Happens again in Bakuryuu Sentai Abaranger; Maharo (who was a dragon for the villians before her Heel-Face Turn) makes a deal with Abarekiller and rejoins the villians with all of her former power restored. The heroes quickly learn that this is all part of her plan to A) free Asuka from the dark armor and B) get close to the Big Bad and wait for a chance to strike. The first half of her plan goes smoothly, but not the second half.
And in the other hemisphere, Power Rangers Operation Overdrive involves a Fake Defector plot when the Rangers need to acquire a Houou statute from a villain faction.
Done in The Unit, which takes it pretty far... Sam McBride, to find Big Bad Leon Drake, "attempts" to rape Bridget Sullivan (who isn't in on this), engages in a shootout and kidnaps Molly Blaine. The result of this is Molly separating from Jonas and Jonas telling Colonel Ryan that he's no longer fit for command, to which Ryan eventually agrees, taking a promotion.
It is revealed late in season 4 of Farscape that Scorpius has pulled one of these on the Scarran emperor. For a time, it is unclear if his devotion to the Peacekeepers was the real fake, but when he helps destroy the Scarran's evolutionary edge, it becomes obvious that his loyalties never lay with them.
Same season, Braca. He appeared to have switched his loyalties to Grayza following the destruction of Scorpius' Command Carrier at the end of Season 3 - gleefully aiding in torturing him for information and even shooting him. However, it was all part of another of Scorpius' plans. Braca even acted as The Mole for Scorpius while the half-breed was hiding out on Moya.
The Doctor himself does it in "The Almost People" when he briefly pretends to be in on the doppelgangers' murderous plan so that he can undermine it later.
Canton Everett Delaware III in between "The Impossible Astronaut" and "Day of the Moon", to get Amy, Rory, River, and the Doctor all back in one place.
Mulder of The X-Files does this in season five's "The Pine Bluff Variant". He even had to hide it from Scully, who was not pleased that she was kept out of the loop.
During the oft-maligned Leviathan story arc of the cult classic Gothic soap opera Dark Shadows, several characters who had managed to resist or overcome the power of the Leviathan cult later pretended to fall back in with said cult in order to help take it down.
In an episode of Arrested Development, Michael and Gob formulated a plan to get back at Oscar for his cruel pranks on them as children which drove the two brothers apart. Gob immediately went to Oscar and told him the plan, or so it seemed. Gob's defection was fake and part of the plan from the beginning. It was believable because Gob would have had no qualms about betraying Michael for real.
Jane attempts this in the fourth season finale of The Mentalist, hoping to convince Red John that he'd truly given up and was willing to join him. He even goes so far as to pretend to kill Lisbon and bring her head to Red John as a gift. Needless to say, things didn't go as planned.
Steve Jinx in Warehouse 13 is fired after pulling a gun on Mrs. Frederic when she insisted on torturing a detainee. He's then recruited by The Dragon to work for the Big Bad. The whole thing was planned by Mrs. Frederic and Jinx off screen, and they were the only ones in on it until Jinx revealed the plan to Claudia late in the game.
In The Aeneid and The Odyssey Sinon surrenders to the Trojans, after claiming to have defected from the Greeks, for the express purpose of convincing the Trojans that the Trojan horse was a gift. He also added the detail that they under no circumstances should bring it into the city because then it would make the city invincible. You know the saying "never trust Greeks bearing gifts?" In context that's actually about Sinon in specific. Older Than Feudalism.
In the Biblical Book of Judith (considered Apocrypha by Jews and, following them, Protestants), our heroine convinces the Assyrian general Holofernes who is besieging her city that she will betray the Jews to him. Then she waits for him to get drunk and does this.◊
In the Battletech universe done by Justin Allard whose fall from grace was manufactured to give him credibility with Capellan Confederation
In Warhammer 40,000, the Alpha Legion's true allegiance is to the Emperor, in the most twisted way possible.
Metal Gear Solid 3: Snake Eater is a deconstruction of the trope. The Boss defects to the side of Big Bad Volgin, but after the US is caught with one of its planes in Soviet airspace during one of Volgin's attacks, the American top brass shifts the blame for the incident onto The Boss to cover their own asses, and her defection suddenly becomes a one-way street, making it necessary for Snake to kill her in order to prevent World War III from erupting. Tragically, Snake learns her defection was fake only after her death.
It gets even worse in Peace Walker, when it is revealed near the end that Not only was the launching of the nuke planned, but the true objective of both the Virtuous Mission and Operation Snake Eater was to kill off The Boss from the start by a single deviously cunning strategist. Coldman even admits (with a sickening amount of glee and satisfaction) that he was the one who set The Boss up.
In every single Metal Gear Solid game (up through 3 at least), in fact, Revolver Ocelot is a Fake Defector to whomever the current game's Big Bad is. He always, always double-crosses his apparent boss at the end. Even more than that, he's not even typically working for the people he "defected" from, but is solely loyal to the Pro-Big Boss faction.
Other than the Chronic Backstabbing Disorder moments, Ocelot's backstory is that he originally worked for the NSA until he and a colleague of his defected to the Soviet Union. However, it was suggested to be a fake defection due to still working for the CIA.Like mother, like son.
One story option, which will happen most of the time in Tales of Symphonia unless you choose Kratos at the Flanoir doctor scene, features Zelos doing this, in part to get one of the required elements to create the Eternal Sword. In the other story path, he really does join the villains and you have to kill him.
If you piece all the story together, Kratos plays with this trope as well. Ultimately, Kratos doesn't believe in Mithos' vision of the world, but his inherent fatalism — and the knowledge of what happened last time he tried — keeps him from truly abandoning the Cruxis leader. This leads to Kratos playing the two of you up against each other and helping in his own fashion, hoping that ultimately Lloyd will win so Kratos will be able to die. He'll never actively oppose Mithos unless you choose him in the aforementioned scene, however.
Jowy Atreides from Suikoden II is an archetypical example of this trope (and a classic Starscream. Originally the main character's best friend and ally in the oncoming war against the demonic and psychopathicPrince Luca Blight (not the Luca above), he betrays said best friend and joins up with Luca, believing that the only way to stop him is from the inside. His plans are successful and in the end of the game, you can choose whether to fight him to the death of not.
Sialeeds does this when she betrays the heroes in Suikoden V for the same reasoning as Jowy's: kill the corrupt people of the Senate from the inside so that no one will oppose the Prince and Lym.
In Lunar: The Silver Star, when the party obtains the Dragon Helmet, Nash betrays them and takes it to Xenobia. He couldn't explain beforehand that he was planning to double-cross the Vile Tribe, and he returns injured but with the Dragon Helmet intact. This is altered in the remakes.
In Mass Effect 3, the cloned Rachni queen, assuming you killed the real queen in 1 will pull this on you just to get access to your big emergency superweapon project, then sabotage the project on behalf of the Reapers. The real queen, should she still be alive, is genuinely on your side though, and her brood of worker drones will provide you with more War Assets than some fleets.
Darth Maul attempts to do this in Darths & Droids, pretending to betray Palpatine for Nute Gunray. Unfortunately, Gunray sees right through it.
In The Gamers Alliance, Briss betrays the Grand Alliance and escorts one of the heroes' enemies to Trinity Gask where he hopes to meet with Big BadGlaurungLosstarot and gain her trust. However, it's only an act on Briss's part because he's still secretly loyal to the heroes and see his defection as the only means to appear to genuinely join the villains and gain their trust so that he can proceed with his plan to gather intel. Things get a bit more complicated for him when he is reunited with his old fling Tiyana in the city and learns that she is working for the villains and is pregnant with his child.
In the Gargoyles episode "Revelations," as an initiation into the Illuminati, Matt Bluestone leads Goliath into a trap set by Mace Malone at the Hotel Cabal, where they intend to break him mentally and then interrogate him. Matt ultimately reveals his true intentions when he saves Goliath from being outright killed by Mace. Viewers learn in the end that Goliath was in on it from the beginning. Additionally, Matt gets into the Illuminati regardless, as the only person who realized his intentions was Mace, who is left trapped in the hotel for his failure and driven insane.
In "Protection," Elisa goes undercover as a Corrupt Cop to get evidence on Draco's protection racket. Out of the loop, Goliath and Broadway notice what appears to be strange behavior and are confused by what's gotten into her. However, they pick it up fast enough and even join in the deception to help bring Draco down.
The Codename: Kids Next Door ep "Op FUGITIVE" is mistakenly thought of as one by many. Details under What Happened to the Mouse?. "Op MAURICE" is a genuine one, with the Fake Defector being a chicken-pox researcher kidnapped by Cree. As the quotes page reveals, Chad (Numbah 274) was this all along in "Op T.R.E.A.T.Y.".
Op UNDERCOVER has Lenny of the Delightful Children defect to the KND to supply them with information. Of course it's revealed he is actually a Fake Defector who did this to steal some valuable intel from the KND. THEN it's revealed the KND knew from the start he was this, and the "intel" he stole was actually a bomb that blows their coffee-drilling oil rig to smithereens.
There was one of these in Beast Wars, with Rattrap pretending to join the Predacons. Ironically, in the first season of the sequel series Beast Machines, he actually goes over temporarily. Well, sorta... Megatron was insanely vulnerable, and he made an agreement to make sure the heroes DON'T kill Megatron. Ironically, Megatron keeps his word, outfitting the weaponless Rattrap with MASSIVE weapons. It appeared he even would have let Rattrap keep them when the battle continued, except Rattrap refused the weapons. (Turns out Rattrap's a HACKER now.)
Autobot Punch of Transformers Generation 1. He changes his robot mode's appearance to infiltrate the Decepticons as "Counterpunch."
On Exo Squad, Marsala does this once. Oddly, in a much later episode, the Neo Megas sorta pull one on him.
Homer: He's about to learn the most important lesson in the music business: don't trust people in the music business.
Tiny Toon Adventures, of all shows, pulled this off with Plucky Duck. With Acme Looniversity due to play its hated rival, Perfecto Prep, in the championship game, Plucky seems to betray his Looniversity friends and begin giving Perfecto Prep inside knowledge of the Looniversity's game plan, up to and including a copy of their playbook. On game day, Plucky gives Perfecto Prep advance knowledge of the plays the Looniversity plans to use...and the Prep students suffer a humiliating defeat. In the end, it's revealed that Plucky was made into a Fake Defectorto sabotage Perfecto Prep's attempts to cheat.
In the third episode of TaleSpin's pilot series, "Plunder & Lightning", Kit Cloudkicker pretends to betray Baloo, Molly and Rebecca in the Iron Vulture to his former mentor Don Karnage, to allow them to escape. The fake deception is so convincing and surprising that Baloo believes himself betrayed, and angrily leaves Higher For Hire with the Sea Duck to Louie's to party his cares away, until he hears Kit desperately radio for help in the fourth episode.
Watching that scene, it seems that the only reason Karnage bought it was because Baloo himself bought it... he seemed fairly skeptical, then looked at the unfeigned rage on Baloo's face and that's what sold him on it.
Both Cyborg and Robin do this in Teen Titans. Robin was less successful, as Slade has already figured out who "Red X" is.
An episode of Cyberchase features Digit pretending to have gone bad again and being rehired by Hacker, in order to prevent Hacker from finding the secret of invincibility.
The Adventures of the Galaxy Rangers episode "Renegade Rangers" was all about this trope, with the heroes pretending to go criminal in order to get to the Black Hole Gang. MaCross (second in charge) never believed it for a second, but Daisy O'Mega, a former Bounty Hunter who went renegade herself, is taken in by the ploy (and Shane's Chick Magnet ability).
The titular heroes in Centurions did this in two episodes, "Max Ray... Traitor" and "Cyborg Centurion". A guest star used the same ploy in "Atlantis Adventure".
Duke from The Mighty Ducks feigned returning to a life of crime after an old member of his mob came to Earth to find out what he was stealing, and why, for Dargaunus.
The G.I. Joe two-parter "The Traitor": When Cobra learns that the Joes have developed an armor treatment that makes their vehicles impervious to their weapons, the Twins approach Dusty and offer to pay his mother's medical bills in exchange for the formula. At first, Dusty only gives them information on the Joes' strategy to stop their next attack, but as the expenses pile up, he eventually steals the formula and turns it over to them, which is followed by Duke being knocked into a coma when a "trap" he and Flint set for Dusty backfires and he's trapped on an oil rig that explodes. Dusty is convicted at court martial and given a life sentence, but Cobra attacks as he's on his way to prison and break him out. Fully indoctrinated with the bad guys, he leads two Cobra raids on chemical plants to acquire materials for their mind control gas and even helps capture an entire squadron of Joes. When Cobra Commander gives Dusty the honor of testing the mind control gas on the captured Joes, Dusty instead releases the gas into the main chamber, where it corrodes the armor-treated Cobra vehicles and Mooks, then frees the Joes and escapes. It turns out that the Joes knew the armor treatment was unstable and wanted Cobra to get it, so Duke and Dusty came up with the plan to get the formula into their hands... but nobody else knew that Dusty's defection was all an act. (Also, the chemical combination that created the mind control gas could also be turned into a compound that neutralized the armor treatment, resulting in the hilarious scenes of both tans and uniforms completely disintegrating.)
Rogue does this is Wolverine and the X-Men, but the X-men actually don't believe her at first and lock her up when she comes to warn them about the Brotherhoods plan, until they start doing exactly what she said they would do. This is because it seemed she had done this in an earlier episode only to lure the X-men into a trap. It's ambiguous as to whether she was pulling a Fake Defector all long, or legitimately joined the Brotherhood, only to later change her mind and claim it so the X-men would take her back.
A later episode of Kim Possible refers to this as a "Scenario 17", which Kim's Cousin Larry executed against Professor Dementor with remarkable skill and cunning, only because he believed it was a LARP session for his birthday.
Detentionaire: Lee needs an in on the Down With Lee Club, so he and Cam stage a fight so Cam can gain entrance. It doesn't end up working, as RadCircles tips off the other club members about the ruse.
Sicinnus: the private tutor of Themistocle's children took a message to King Xerxes at the Battle of Salamis advising him to attack. Whereupon the Persian fleet fell into an ambush.
Eric Erickson was an American-born Swedish businessman who pretended to have converted to Nazism in order to visit Germany — and scout German petroleum facilities for targeting by Allied bombers.
This was actually a pretty common phenomenon during the Cold War... in order to diminish the effectiveness of REAL defectors, the USSR sent several false defectors to flood US intelligence agencies with conflicting stories. One famous example came during the Golystin-Nosenko controversy, which ultimately led to the CIA shifting completely away from human intelligence to signals intelligence (e.g. wiretapping, satellite photos, cryptography...)
One reason that the Germans had such poor intelligence in World War II was that one of the finishing instructors in the spy school was actually an MI6 plant, allowing them to pick up virtually every German spy in the first few weeks.
Contrary to popular belief, Vince Russo was not one, but when he left for WCW he was so bad that Vince Mcmahon and others would ask if Russo was still on their payroll.