Literal Change of Heart
Well, as it turns out, you can do exactly that: stick a new heart in your character's chest, and watch him suddenly improve! He'll care more about people, he'll act like less of an asshole, or maybe he'll even take on personality aspects of the person who gave him his heart.
Depending on the genre, this may involve actual heart transplants, but it also counts symbolic hearts (in the case of things that do not literally have the organ, or who want to make the transfer without the Squick involved with a literal heart transplant) or heart-like objects (such as the pneumatic "heart" of a robot). Note that the change of heart is not always for the better. In some cases, the new heart makes you more... well, heartless.
This trope can also be played heroically, if one character nobly sacrifices his or her heart to another character who is not necessarily bad. In these cases, the emphasis may be on the sacrificial giving of the donor, rather than on any potential changes in the recipient's personality. The two versions of the trope aren't necessarily exclusive: if the noble character sacrifices his or her heart in order to rescue or convert an evil character, the "bad" nature of the recipient serves to highlight the generosity of the sacrifice.
- Angel Beats! has the rare heroic example of Otonashi donating his heart to Tenshi after dying.
- In Blue Submarine No. 6, it turns out that the doctor everyone blamed the world's problems on removed his own heart and used it to power the Earth's destabilizing magnetic field. So, in this case, the one who received his heart was the planet.
- This is the premise of Angel Heart: receiving a heart transplant changes the life of an assassin.
- In Busou Renkin, the villain Victor's Start of Darkness began when he had his heart replaced by the experimental Black Kakugane.
- In Science Ninja Team Gatchaman the Condor has to have his heart replaced.
- Dragonheart: Draco gave half his heart to the prince hoping it might redeem the prince from the ways of his wicked family. It didn't work.
- A sort of example, Tony Stark from the Iron Man movie; though the heart didn't actually come from someone (or something) else, getting the arc generator put in his chest did symbolically represent his assumption of the role of Iron Man, thus fitting the trope.
- In the movie Heart Condition, a heart transplant patient ends up stuck with the ghost of the donor, a black man whom the recipient hated because he was racist, and because the recipient a cop and the other guy a lawyer. This of course brings about an emotional change of heart as the movie progresses.
- In Repo Men, the main character only acts like a decent human being after losing his heart and having it replaced with a mechanical heart.
- In Return To Me, Grace receives a transplanted heart that previously belonged to Bob's late wife. Although the movie is mostly not supernatural, there is one brief moment which suggests a connection between Grace and Bob due to the transplanted heart.
- A heroic version occurs in Terminator Salvation. John Conner gets mortally wounded and Marcus makes a Heroic Sacrifice giving him his own heart.
- In Clockwork, by Philip Pullman, a character's mechanical heart is winding down, threatening his life.
- In Howl's Moving Castle (both the anime and the novel) Calcifer had swallowed Howl's heart as part of an old contract between them. The ending involves the heart being returned to its original owner, with some improvement of said owner's character.
- In Meredith Ann Pierce's book The Darkangel, two characters switch hearts in an attempt to save one of them at the expense of the other's life. They both live, because as it turned out, there was a way to revive the vampyre's heart without Aeriel giving up hers.
- The titular warlock in the Harry Potter fairy tale The Warlock's Hairy Heart attempts this. It doesn't go well for anyone.
- One of the older print examples is the Tin Man from The Wonderful Wizard of Oz. Because of the curse on him and the Emergency Transformation he underwent to survive it, he does not have a proper heart. He wants to ask the Wizard for a replacement so he can return to his girlfriend and become a proper husband for her. He succeeds in his task and gets a sawdust and silk one. Too bad his girlfriend was seeing someone else on the side, and got married when he was rusted by the roadside.
- Dr. Suess' The Grinch had a change of heart about stealing the Christmas gifts from Who-Ville after he heard the Whos celebrate anyway and his heart grows three times as large as it was before.
- In the series finale of Smallville, Darkseid takes Lionel's heart out and plants it in the Lex clone's body.
- Occurs in the "Brown Betty" episode of Fringe.
- In Star Trek: The Next Generation, Captain Picard got in a fight with some surly Naussicans as a youth and got stabbed right through the chest. Now he's got an artificial heart. During a near-death experience in a later episode, he was asked by Q if he would like to change that part of his past that lead up to that (and the subsequent malfunction of the artificial heart years later); however, by doing so, he wound up becoming a person who never developed any guts or took any risks.
- Played with in an episode of The Ricky Gervais Show. In the episode, Karl talks about a made-up TV show he came up with called "Look What We Can Do With Science" and has him explaining how he thinks human organs can just be either removed or replaced. He then states that they could replace a person's heart with a pacemaker, but Ricky is quick to point out that they don't really replace the heart with a pacemaker in real life.
- This trope is in one origin story for Xuanwu (Genbu) of The Four Gods: as a mere mortal, he was a ruthlessly brutal butcher until he had a vision where he saw his horrifically disgusting insides being swapped out for much nicer ones. This resulted in him getting religion and becoming a saint, then going back to battle his old insides (which had become monstrous).
- In Kingdom Hearts, people who lose their hearts become, well, The Heartless. So it was fairly surprising to learn that not only had Kairi not become a Heartless, her own heart had been with Sora the whole time.
- And, of course, Kairi ended up returning the favor later on.
- In Brutal Legend, Demon Emperor Doviculus keeps his defeated enemies' (including Drowned Ophelia's) hearts in his chest. It doesn't influence his Neutral Evil morality much.
- Legacy of Kain features the Heart of Darkness, which is heart of the last ancient vampire Janos Audron, ripped still-beating from his chest by Sarafan vampire hunters and christened as such. In the first game, the Heart simply acts as a healing item, but eventually it's revealed that the same heart was used by Mortanius the Necromancer to resurrect Kain, and it's also the only thing that can resurrect Janos himself.
- In Team Fortress 2's "Meet the Medic" video, the Medic accidentally destroys the Heavy's heart when trying to attach the Ubercharge device to it. He substitutes a larger heart from a "Mega Baboon" for it instead.
- Tales of Vesperia: Raven's heart was replaced with a modified blastia after he died during the Great War, which took place 10 years prior to the events of the game itself. It not only sustains his life, it allows him to unleash the very same power as an offensive weapon; as seen during his Mystic Arte.
- During the Old World Blues DLC for Fallout: New Vegas, the player has his/her heart (and brain, and spine) removed and replaced by a synthetic one. Later on, you have the choice of keeping your powerful synthetic organs or putting your old ones back in. (Upgraded with a bit of tech, of course)