The Damsel in Distress is in the clutches of the villain, but the villain decides that chains, cages, cells, and other ways of containment are too unreliable. The hardest chain for The Hero to break while freeing this prisoner is in the damsel's own mind. Thus the villain hypnotizes the damsel. As with Save the Princess, this need not be a princessor even a lady.
The motives can be varied, but the two most common are (1) the villain wants the captive for a lover (this is easier than coercion), and (2) to make the captive betray the hero once "rescued". How the damsel is controlled can be done by straight-up hypnosis, possession or a Love Potion (for either motive).
Note that this rarely works well, save for cynical enough stories. The Evil Overlord List would likely recommend this be done just to get her to cooperate in that quiet civil ceremony so you can marry her with as little fuss if possible. Anything else is just asking for trouble.
Of course, Freud Was Right, and such brainwashing is inevitably Fetish Fuel for some and subject to Rule 34.
Compare Hypno Fool, Stockholm Syndrome (doing this over long term), More than Mind Control, I Have You Now, My Pretty, Hypno Trinket.
Tuxedo Mask is the number one victim throughout the series. Queen Beryl brainwashes and corrupts him in every adaptation of the storyline, with his rescue being a pivotal element of the climax of that story arc. He's a victim again in the Black Moon arc of the manga (this time by his corrupted future daughter Black Lady), and yet again in the Sailor Stars arc of the manga (by Galaxia). He's also brainwashed in the Nehellenia arc of the Sailor Moon anime.
Chibi-Usa famously becomes Black Lady when taken by the Black Moon Clan in both the manga and the anime. In the anime, Prince Diamond attempts this on Sailor Moon, but fails.
Sailor Mercury is turned into Dark Mercury in Pretty Guardian Sailor Moon and serves the Dark Kingdom. In the anime, a filler episode showed a Monster of the Week attempting this, but as it was a filler episode, the attempt eventually fails.
This is attempted in the Dead Moon Circus arc of the manga on Makoto, Ami, and Rei. Each time fails.
And in the Sailor Stars arc of the manga, after Galaxia succeeds in eventually killing all of the Sailor Team and Mamoru, she revives all of them as members of Shadow Galactica and forces them to fight Usagi.
Done to Ayeka in the Tenchi Muyo!manga. And almost done to her in the OAV, but it didn't work.
And to Ryoko, in the original OAV.
An early example of the trope appears in Toei Doga's Majokko Megu-Chan (1974). Recurring villain pervert Chou-san designs a magic clock rigged to hypnotize people teenaged girls into removing all their clothing. Falling under its spell, Megu gets down as far as her her bra and panties◊ before Non nukes the device.
It is worth noting that Megu is a potential heir to the throne of the Witch Kingdom, making her a literal hypnotized princess in this case.
Yu-Gi-Oh! GX: Saiou tries this with Asuka Tenjouin in a last-ditch attempt to get the keys to the SORA Satellite from him. Saiou (or more precisely, the Light of Ruin) didn't care about Juudai anymore at that point.
The original Yu-Gi-Oh! has that happen with Anzu Mazaki durring the Battle City arc, meaning so far, the only girl to be the favorite ship with the main character of a Yu-Gi-Oh! series without being under some mind control is Aki Izayoi, but that show is still being made.
As of episodes 60 and 61, Aki became a member of this "illustrious" group.
Also happened at least once with Mokuba.
And with Jounochi in the same incident that got Anzu hypnotized. Then his Brainwashed and Crazy self duels Yugi. Curiously, Mokuba was not hypnotized that time... because he was caught alongside Anzu and she actually got him free. Kaiba frees the captured Anzu at the end of the duel to repay her for it.
Used in Sensual Phrase, when Aine is brainwashed by Tomoyuki into believing that she's his dead sister Yumi and forgetting about her Bastard Boyfriend Sakuya. He manages to defuse the brainwashing, but almost dies while trying.
Used more than once in Ranma ˝, on different characters. Most memorably, Shampoo once wipes Akane's memories of Ranma to get him to fight her, and later uses a literal Red String of Fate to brainwash him into marrying her.
In the PC Engine game, Toraware no Hanayome, Ranma herself is subjected to hypnosis and dressed in royal regalia in order for her to marry the Bear Prince. Akane and the others have to race to rescue her before she imbibes a drop from a flower that would permanently and irrevocably make her fall in love with this prince.
Gender flipped in Märchen Awakens Romance anime where the Big Bad Phantom in a last ditch effort to get back at Team MAR after defeating him officially in his tournament, he kidnaps Alviss. However, Alviss lets him get captured so he can finish him off once and for all...but Phantom predicted this and easily beats him before hypnotizing him thus does the Rescue Arc begin...
Played more straight with Snow towards the very end of the anime. She was brainwashed to try and erase Babbo's memory by her stepmother. She was also apparently brainwashed to forget all the visions of Tokyo she had when she was younger.
In Nintendo Power's Super Mario Adventures comic, Bowser has Magikoopa do this to Peach so she would marry him (shown in the image above) — though in this case, it was a last resort after he tried everything else and the willful Peach was too much.
And in one issue of the Nintendo Comics System, Peach is once again mind zapped, this time by a Pidgit. This notably actually ends up being a hindrance to Bowser - while in her hypnotized state, Peach wrecks this blimp that he's driving.
The Mockingbird / Phantom Rider storyline in West Coast Avengers. When Mockingbird found out what he'd done (and that he'd raped her)she let him fall to his death.
Happened to Ms. Marvel at the hands of Marcus, who actually got her pregnant, and made her give birth to... himself? Fridge Logic (and a little perspective from a different writer) made this story a little unbelievable.
"Three Old Men of Aran" did this to Aaricia in Thorgal.
A rare example of a protagonist doing this is found in Harbinger. Telepath Peter Stanchek uses his powers to make his childhood crush Kris fall in love with him. The ugliness of this, and Kris' subsequent psychological trauma and feelings of violation, are not glossed over.
In the Spider-Man comic strip, he was helping heiress Muffy Ainsworth with what she thought was a vampire. She gets kidnapped and then hypnotized so that she would agree to be one of his brides. It was staged. The vampire was really a washed-up actor trying a really weird plan for a comeback. Muffy being hypnotized was the only part not faked, just not with vampiric powers.
Jafar attempts to do this to Jasmine in Aladdin, but when the Genie tries to explain he can't, she fakes it to distract Jafar. Jafar never attempts to use the Mind-Control Device he already has to Hypnotize the Princess, but it's vaguely implied that said snake staff only works on weak-willed people (e.g. the Sultan) and Rebellious Princess Jasmine is too strong-minded. It also only seems to affect the victim while he's actually holding it in their face, making it impractical for long-term work. So basically, it's a SithMindTrick.
Sleeping Beauty: When the glowing green orb appears in Aurora's bedroom as she cries, the light makes her suddenly stiffen and look up. Her eyes are glazed, and she stands up very fluidly, following the orb single-mindedly. When the fairies call out to her to not touch anything, it momentarily breaks the spell, before Maleficent's voice lulls her back in, and she touches the spinning wheel's spindle.
Darkseid does this to Kara/Supergirl after capturing her in Superman/Batman: Apocalypse to make her his dragon, leading to a climatic battle between her and Superman in Darkseid's throne room. Of course, Evil Kara, instead of the normal street clothes she wore previously, is now clad as a hot teenage dominatrix.
Films — Live-Action
The Brides of Fu Manchu might be more accurately titled: Hypnotize the Princess: The Movie. Although Fu displays no apparent interest in the women as literal "brides"; they are kidnapped to force cooperation with their scientist fathers.
Happens to poor Dale Arden at least twice in the classic Flash Gordon movie serials: Ming the Merciless plans to marry her with the help of his hypnosis machine, and later she's drugged by a tribe of religious fanatics and literally backstabs Flash.
Also occurs to her in the 1980 Flash Gordon movie when Ming hypnotizes her with a ring, causing her to writhe sensuously.
A mild example in Scott Pilgrim vs. The World, where Gideon Graves has implanted a mind-control chip in Ramona's neck, making her go back to him and sit placidly beside his throne while he fights Scott. The film even gives a Shout-Out to the 1980 Flash Gordon mind-control ring by using its sound effect when Gideon kisses Ramona's ringed hand during the Katayanagi Twins battle.
Lazar's "mothers" put a spell on Belladonna in Your Highness to make her "submit" to him so that he can impregnate her with a dragon. Hilariously, all the spell does is make her horny, which freaks Lazar out to the point where he has difficultly performing. Fortunately, this gives the heroes plenty of time to kill Lazar's minions.
The 1962 version of Jack the Giant Killer featured the villain hypnotizing and turning the princess into a witch, thus ensuring that even if Jack managed to save her, it wouldn't be nearly as hard to get her back the second time.
Space Balls plays this for laughs, like every other trope at its disposal. Lord Helmet uses The Schwartz to trick Princess Vespa into thinking he's her father. He only keeps it up long enough to capture her.
Lord Helmet: Fooooled you!
The 1985 film Legend not only subverted this trope but subverted it twice in a row with the same character, keeping the audience guessing until the end whether or not it had actually worked.
This happens to Eilonwy in The Castle of Llyr. Somewhat unusually for this trope, her captor is female and kidnapped her for entirely pragmatic reasons.
Was used by one of the good guys (technically an Anti-Hero, but still) in one of the Dragonlance novels. In order to unite a country against the forces of evil, he forced a witch to use her magic and make a princess fall in love with him. It works and the kingdom is saved, but the Anti-Heroiscalled on it later.
In Edgar Rice Burroughs's Thuvia, Maid of Mars, Tario uses this on Thuvia, convincing her in moments that he is friendly, and that she is in love with him. However, it doesn't last long — either his smug expression reminds her, or she gets a more powerful counter-suggestion from Carthoris.
Rather unconventional example, but Moghedien does this to Elayne. (And Nynaeve, but she's not a princess.)
Graendal does this as a rule - she likes her pets to be compliant.
To be fair to the princesses of the Wheelof Time-verse, the Forsaken use Compulsion on just about everyone; the princesses listed here just happened to be the ones in their way at that point in time.
In The Silver Crown, the black metal stuff is being used to brainwash just about everyone, but it only applies so long as you're near the stuff. The heroine's apparently immune. She breaks into the fortress to free her friend, and manages to get free with both her friend and a new girl who's also apparently immune. But when they get back to the place where they hid the silver crown, her friend turns traitor! Turns out he was wearing a belt made of the black metal, and calls in the enemies to capture them all.
In The Silver Chair, the Big Bad had Hypnotized Prince Rillian into becoming a sort of Empty Shell without any memories of his past life as the Crown Prince of Narnia. The spell wears off temporally every night, thus Rillian ends tied up to the eponymous Silver Chair when that happens so he won't escape. The Power Trio frees him, though.
In the sixth Book of Lost Swords: The Mindsword's Story, Murat uses the Mindsword to compel Princess Kristin, among others, to love and adore him. When he loses the Mindsword and is killed, the effect wears off on all the other victims after a few days, but Kristin remains devoted to Murat indefinitely afterwards. Curing her of this drives the plot of the seventh book.
In Andre Norton's Ice Crown, Roane sees Princess Ludorica, before defiant, being ushered about by her kidnapper without resistance.
Morgana does this to various characters in Merlin.
Kreel does this to Vara at least three times. One of those times he isn't even anywhere near her, he's just projecting dreams into her mind.
In a subversion, Kalem does this to Will in the first episode, to make him run instead of facing Xax's men. Will's not very happy it, because it means leaving his parents behind.
A variant in the Expert Dungeons & Dragons module The Curse of Xanathon: an evil spellcaster can't take over the duchy by marrying the ruler (because they're both guys and it's not the Netherlands), so he ensorcels the duke into making insane proclamations which will destabilize the kingdom and facilitate a more conventional Invade-the-Kingdom plot. The brainwashed duke doesn't actually fight the heroes, but some of his proclamations may get the guards to do it for him.
Queen Sindel from Mortal Kombat is brought back from the dead and brainwashed by Shao Kahn into being his queen. She pulls a Heel-Face Turn after her daughter Kitana gets to her.
In a Super Smash Bros.. Melee Event Match, Mewtwo hypnotizes Princess Zelda to fight you until he shows up.
And not only does something similar happen to her in The Legend of Zelda: Spirit Tracks as well, there's also the second-to-last boss battle, where Ghost Mice repeatedly possess Zelda, while she is possessing a Phantom.
In Xenogears, Miang uses a particularly dangerous mental conditioning on Elly to get her to sabotage the party. After Elly betrays her team, Citan reveals that he was aware of the brainwashing but let her carry out the orders... because to prevent her from doing so could kill her.
Also attempted with Princess Julia, Deirdre's Dark Magical Girl daughter and sister of said vessel, Prince Julius. It fails this time, though.
In Thracia 776, Mareeta is a variation. She's actually not hypnotized, but forced to use an Evil Weapon that takes over her mind. She gets better, and ultimately said Evil Weapon becomes her personalised sword.
Don't forget Ninian in The Blazing Sword, brainwashed and forced to open the Dragon's Gate and call a Fire Dragon by Nergal. Good thing her younger brother Nils showed up before it got very far and debrainwashed her! And later, Nils himself almost goes through the same thing, but fortunately it fails.
In Tales of Vesperia, there is a part where Estelle is controlled by Alexei and forced to fight the party and then Yuri one-on-one, but Estelle still seems to be well enough to beg Yuri to kill her and to respond when Yuri actually chastizes her...
A little-known example from Final Fantasy IX: During the fight with the second Black Waltz (when the party attempts to leave the Village of Dali), the Waltz will not attack Garnet/Dagger. This might sound as though it makes the fight a Foregone Victory, but no. Should the other characters be defeated, the Blazk Waltz will use "Hypnotise" on Garnet, and the battle ends.
Prince of Persia 2: The Shadow and the Flame begins with the vizier doing this, and taking it a step further by impersonating YOU!
The NES version of Double Dragon III has the player face off Queen Noiram, a possessed Marion at the end. The game never really explains who kidnapped her and how she ended up being possessed (due to the fact that in the Japanese version, the final boss was actually Cleopatra, not Marion).
World of Warcraft had this. Princess Moira Bronzebeard, daughter of the Dwarf King, marries an evil Dark Iron Dwarf Emperor. Her father says it is because he put a spell on her. Nevertheless, once you kill the Emperor and rescue the princess, she remains loving to his memory. The game notes that she "may still be suffering from the residual effect of the Emperor's spell." Nearly five years later, it would turn out she truly did love the emperor (He treated her well, and she was estranged to her father because he wanted a boy), and still mourns him.
In the Elven campaign of Heroes of Might and Magic IV, the hero's girl has been kidnapped by his evil rival, who puts her under a mind control spell. The only way to free her from it (even killing the Big Bad won't do it), is to construct the Mirror of True Love. All she has to do is look into it to see her true love's face, and the spell will be broken. However, the Big Bad mentions in a not-so-subtle manner that they have already consummated their "relationship."
A common hazard in Mass Effect, the villains being cyborg demigods capable of indoctrinating most younger beings. This is played most straight in The Arrival, where Shepard is sent to rescue an Alliance scientist from the Batarians at Admiral Hackett's behest. She was indoctrinated by the Reaper artifact she was studying before she was captured, and betrays Shepard soon after her rescue.
Code Lyoko: It is a common tactic for XANA to mind-control Aelita into doing his binding. Once in Season 2 through a Mind-Control Device, several times in Season 3 (and once in Season 4) with the Scyphozoa. And sure, her nickname is "Princess".
The Fairly OddParents. TimmyTurner wished: "I wish TrixieTang loved Timmy Turner." as his second Norm the Genie wish. Norm the Genie made lots of Timmy Turners appear, listed where they were from, what their exact name was and made Trixie Tang kiss them all. Give Norm a Made of Win. Strangely, he phrased the wish that way to be more specific to minimize Norm's meddling, but just saying "I wish she loved me." (while indicating Trixie) probably would have worked far better.
Dr. Wily does this in the Mega Man cartoon: he gets another robot (Dr. Petto) to make Megaman doubt his humanity, then allow him to work on him. Wily places a chip in Mega, then toys with his mind for a while before outright taking control of him.
Ming the Merciless pulls this on Dale in The New Adventures of Flash Gordon, despite never having displayed mind control powers before or since. Then he actually "marries" her before Flash and company come to her rescue, although it doesn't have any lasting effects. (This was likely a deliberate homage to a similar sequence the first Flash Gordon movie serial.)
In Wakfu episode 6, Evangelyne is hypnotized before being captured by Vampyro, who plans to use her body as a host for the demon Ombrage.
In an early episode of the X-Men cartoon, Scott and Jean are kidnapped by the Morlocks. When the other X-Men come to rescue them, they run across a hypnotized Jean.
Happens to her in X-Men: Evolution too, thanks to Mesmero. She's not the only one, either: Kurt, Kitty and Evan were under the same treatment.