Of all the organs in the human body, the brain is the most important. Not only is it what's animating the body, but it's where the consciousness is stored. So of all the organs to thieve, it might be the most valuable. Maybe the Mad Scientist wants to do experiments on it or forcibly put someone's brain in a fancy jar. Or somebody's looking to take that body for themselves and needs a vacancy. Whatever the motive, the pounds of protein in the skull's got a target on it.
As nobody knows how to sustain a Brain in a Jar or safely switch it with another body in Real Life yet, it tends to not show up in the high-end of Mohs Scale of Science Fiction Hardness, unless its a dead brain being taken for study and autopsy without consent. Lighter and more comical portrayals have the brainless body survive without a brain, just acting like an complete idiot.
A sub-trope of Body Snatcher and Organ Theft, though the latter generally concerns organs that can be transplanted in Real Life (usually kidneys). Compare and contrast Brain Transplant, which is usually when the brain is transplanted consensually, but they can overlap. If the only reason the brain is stolen is to be eaten, it's Brain Food, and if the brain is physically tampered with, it's Lobotomy. Can be used to perform a Kill and Replace
- Dragonball Z The Worlds Strongest: The plot of the movie is that Doctor Wheelo is trying to replace Master Roshi's brain with his own, so that he can have the body of the world's strongest warrior and escape his status of being a Brain in a Jar. He changes his target to Goku when he learns he's the actual world's strongest warrior. It's not quite clear how he's expected to fit inside Goku's cranium though, since his brain is huge.
- JoJo's Bizarre Adventure
- A full-head version happens at the end of Phantom Blood, where Dio Brando as a head decapitates Jonathan Joestar and puts his head on his body. He's sealed away for almost a century until the events of Stardust Crusaders where he becomes the Big Bad again using Jonathan's body.
- Inverted in Battle Tendency: After having his body destroyed, the brain of Esidisi pulls off a desperate last move of hijacking Suzi Q's body so he can give his allies enough time for their own plans.
- DC Comics:
- In Seven Soldiers of Victory, Neh-Buh-Loh was charged with killing Misty Kilgore and bringing her brain back to Queen Gloriana. Unable to do it, he instead brought stole the brain of a telepath and offered that to Gloriana instead.
- Superman: This is the Ultra-Humanite's gimmick-originally a Genius Cripple, he surgically transfers his brain into better bodies. He normally uses an albino gorilla, but has also put his brain in an actress and a T-rex. During Power Girl's series he tried to put his brain in her body to gain her Kryptonian powers.
- Marvel Comics:
- After Professor X dies, the Red Skull graverobs his brain and revives it so he can put it in his own head. This gives him the professor's psychic powers, and transforms him into the Red Onslaught.
- In New X-Men, Martha Johansson was an innocent mutant runaway who was kidnapped by the U-Men, who removed her brain from her body and placed it in a tank in order to force her to do their bidding.
- The Brain That Wouldn't Die: A mad scientist tries to keep his decapitated girlfriend alive by transplanting her brain (and her head) onto a new host, despite her objections to the horrifying process.
- In The Man with Two Brains, Dr Necessiter opportunistically removes the brains of a serial killer's victims and keeps them alive in jars.
- In Muppets from Space, the government threatens to suck Gonzo's brain out with a huge machine that is like a cross between a drill and a vacuum cleaner (and is big enough to require a cart). Cue Gonzo screaming "NOOOOOO! I NEED MY BRAIN!"
- Cthulhu Mythos: In the H. P. Lovecraft story The Whisperer in Darkness, the alien Mi-Go can perform operations on living creatures (including human beings) to remove their brains. The brains are placed in metal "brain cylinders", which can have other devices attached to them to allow the brains to listen, see and speak. In the story, after capturing a man named Akeley, the Mi-Go perform a brain-removal operation on him and store his brain in a cylinder.
- Star Wars Legends: The B'omarr Monks are brains in spider-like droid jars who become so to find enlightenment. While usually willing, in Tales from Jabba's Palace where they turn their deal with Bib Fortuna and put his brain in said spider droid-jars. Bib eventually escapes his predicament in X-Wing Series by stealing a rival's brain and putting him in a Brain in a Jar.
- Warhammer 40,000: In the Forges of Mars trilogy, the robot Galatea steals people's brains in order to add them to its Wetware CPU network. It does this on a regular basis, typically to replace those brains that have gone crazy from decades of being trapped in a glass jar while a psychotic robot plunders their mind for information. Linya Tychon becomes Galatea's latest victim in the second book.
- Doctor Who:
- While the Cybermen have always been about Unwilling Roboticisation, the Cybus Cybermen introduced in "Rise of the Cybermen" cut out the middle man by removing the brain alone and putting it into a Cyberman shell. John Lumic starts the process by having people forced into factories where they can surgically remove the brain.
- "The Return of Doctor Mysterio" involves alien brains who are trying to take human bodies for themselves. They do this to take over planets, and have tell-tale scars from it.
- Metal Hurlant Chronicles: The pilot episode has a dying tyrant hold a gladiator tournament to pick his successor. The final scene reveals he's actually holding the tournament to pick a new body for his brain to be transplanted into.
- The Star Trek: The Original Series episode "Spock's Brain" has a race of aliens disable the Enterprise and steal Spock's brain to lead them. Captain Kirk then leads a mission to retrieve and reinstall it.
- Tin Man features Glitch, formerly the queen's mannerly chief advisor and inventor, who has been reduced to a shabby and absent-minded wanderer since villain Azkadellia had half his brain removed and stuck in a jar so she would have unhindered access to his intellect to further her plans.
- Pathfinder: The neh-thalggus, or brain collectors as most others know them, are one of the numerous races of monstrous aliens that make up the star-faring empire knows as the Dominion of the Black. Their name comes from their ability to remove brains from living humanoids and store them in special blisters on their bodies, which they wire into their own nervous systems in order to increase their intellect and brainpower. The captured brains are still alive and aware through this process; the neh-thalggus don't care. Old and powerful neh-thalggus can absorb their stored brains to transform into larger, stronger yah-thelgaads; these can only store six brains at a time, but can collect the brains of non-humanoid organisms as well.
- Fallout: New Vegas: At the beginning of the Old World Blues dlc, the Courier's brain along with their heart and spine are removed and replaced with advanced technology.Near the end of the story the player can choose to keep the implanted technology or have the organs returned with the help of a Auto-Doc Machine.
- In Psychonauts, there is a pepper-like compound simply referred to as "Super-Sneezing Powder" that can cause people to literally sneeze their brains out if inhaled. This is the primary tool Dr. Loboto uses to steal the brains off all of the Psychic Children so that he can create Psychic Death-Tanks and conquer the world with them.
- The Sam & Max: The Devil's Playhouse episode "The Tomb of Sammun-Mak" ends with Sam discovering someone has stolen Max's brain. The following episode, "They Stole Max's Brain", has Sam go on a quest to recover it.
- Wolfenstein: The New Order: At the beginning of the game, William "B.J." Blazkowicz, Fergus Reid and Private Probst Wyatt III take part in a massive Allied raid against a massive fortress on the coast of the Baltic Sea, reported to be the hideout of and weapons laboratory run by his arch nemesis, General Wilhelm "Deathshead" Strasse. After an infiltration attempt goes wrong, the three are cornered and captured by Deathshead and his Super Soldiers, where Blazkowicz is presented with a difficult moral choice: between Fergus and Wyatt, he must pick one of them to sacrifice to his nemesis' sick experiments or he will have everybody in the room killed. Whichever is chosen has their brains painfully (and very graphically) removed and used as the brain for the game's penultimate boss the Prototype Robot.
- American Dad!: Klaus's backstory is that he was an Olympic Ski-jumper for East Germany, before the CIA took his brain and put it in the body of a goldfish so he wouldn't win the gold medal. The CIA still uses this technology throughout the series, using it to put the brain of Reginald into the body of a koala. Later on Klaus pulled the same stunt on Stan so he can have a human body and Stan gets to see what life is like as a goldfish, but they end up switched back at the end.
- The Avenger Penguins episode "The Quantum Mechanic" had Bluey's brain stolen by Caractacus P. Doom and his henchman Harry Slime. By the end of the episode, Doom and Slime are punished by having their own brains removed, with Doom's brain angrily chasing after Slime's brain while they're both in a giant jar.
- Batman: The Brave and the Bold: In the opening segment of "Night of the Huntress", Grundy's goons are about to cut open somebody's head because "the boss wants a bigger brain". An unusual example in that Grundy intends to replace his own brain rather than transplant it into a new body (it doesn't make much sense, but Grundy isn't very bright and his henchmen presumably know better than to raise objections).
- In Ben 10: Omniverse, "For a Few Brains More" has Albedo removing the brain of Azmuth, his former mentor and the Universe's Smartest Mind, so that he can use his intelligence for himself. However, due to the fact Azmuth is a Galvan, he possesses two brains with only his "main brain" being taken while his "pre-brain" is still in his body to handle bodily functions. His main brain is also able to survive separately from the body and audibly speak, while the pre-brain leads Azmuth to be simplistic and childish.
- Darkwing Duck has an episode where the Evil Chancellor of an alien princess wanting to make Launchpad her husband planned to replace Launchpad's brain with a robotic one. Darkwing discovers the plan just in time to be used as a guinea pig for the procedure, waking up as a Brain in a Jar. In true cartoon fashion, this does not stop Darkwing from effecting an escape and attempting to kung-fu fight his own body to save the day.
- Futurama: A robotic version happens in "How Hermes Requisitioned His Groove Back". After catching Morgan Proctor sleeping with Fry he tries to go public with it but she steals his memory drive, effectively his brain. This reduces Bender to monotonely asking if he can bend girders, and the crew has to go to Central Bureaucracy to get the chip containing his personality back.
- Invader Zim
- "Dark Harvest" has Zim steal organs in general so he can pass as human, replacing them with stuff. One of which is a brain, which he replaces with a can he found.
- "Bad, Bad Rubber Piggy": Zim accidentally does this to himself. In order to kill Dib before he becomes a threat, Zim throws rubber piggys back in time so they replace things in the past. This backfires and makes Dib more dangerous, so he's forced to abort the plan by sending a piggy with a message not to use time travel to his past self. Unfortunately for Zim that rubber piggy replaces his past self's brain.
- Looney Tunes: Water, Water, Every Hare has the short, greenish Mad Scientist pursue Bugs Bunny, intending to install the rabbit's brain into his Giant Mecha robot. The rascally rabbit defers:
Bugs Bunny: Sorry, doc. I need what little I've got.
- Men in Black: The Series: In the episode "The Quick Clone Syndrome" by Alpha steals Zed's brain to get information about MIB. Zed's body still functioned because of alien tech and his brain is eventually retrieved in the end, though
- The Ren & Stimpy Show: "Ren's Brain" has Stimpy remove Ren's brain to be the centerpiece of his weird brain collection, leaving his body as an idiot. Ren's brain ends up leaving and acting as if nothing happened, only to come home and be pissed off at Stimpy hanging with his body.
- Runaway Brain: In order to get money for a vacation Mickey Mouse ends up working for Doctor Frankenonllie, who forcibly switches Mickey's brain with that of his monster Julius. The result is a forced "Freaky Friday" Flip with Julius becoming a monstrous-looking Mickey, and the actual Mickey trying to get his body back.
- The Simpsons: In the "Treehouse Of Horror II" story "Homer's Nightmare", Mr. Burns steals Homer's brain to put into a robot designed to be the perfect worker. Since this is Homer, that doesn't turn out to be the case.
- Spongebob Squarepants: In "Welcome To The Chum Bucket", Plankton wins Spongebob in a bet, but he ends up being lazy and not doing his job. Frustrated with this Plankton tears out his brain and puts it in a robot body to be a better worker. However he still acts just as much of a Lazy Bum and Plankton gives up, putting his brain back in his body and giving him back to Mr Krabs.