The Hero sees a Damsel in Distress in the claws of someone that they really not should be around - An Evil Sorcerer, a Mad Scientist, The Don, etc. - and immediately assumes that they have kidnapped the Damsel and is holding her against her will. The Hero unsheathes his sword and is more than happy to strike that villain down and ride off into the sunset, both of them on his horse.
There is just one problem: the Damsel is not in distress. Or so she tells him.
She explains that the villain is not so bad, and that he was actually her friend. He would never harm her and the Hero's assumptions that he would just makes him prejudiced. Perhaps the villain had used magic and hypnotic technology to brainwash her so that she would say such things, or maybe she was threatening her and her loved ones if she ever confided in anyone the horrible things that he does? The Hero has long since made up his mind on the issue and he refuses to think otherwise.
When the supposed good guy makes the assumption that the damsel has been gaslit into Stockholm Syndrome in such a manner, then they are either a complete idiot to not see the lack of evidence of abuse, or an Unscrupulous Hero who is out to ruin the Designated Villain's already paper-thin reputation by claiming that he would defile the brain of such a beautiful maiden. Acting upon his impulse to "rescue" her and "free her from this spell".
All attempts will undoubtably fail and he will persist to try new and increasingly archaic methods to prove otherwise. Insane Troll Logic, various holy and magical rituals, fasting and repentance, therapeutic hypnosis, maybe even a little enhanced interrogation to enforce the truth into them one defibrillator to the temple at a time. Eventually he may concede to the fact that she is Not Brainwashed and really is telling the truth. If the denial persists long after it should, it usually delves into I Reject Your Reality Sanity Slippage territory, coming to Believe Their Own Lies in-spite of all of the evidence they find along the way because they are the hero and the hero just knows.
Sub-Trope to Unwanted Rescue. Compare with Condescending Compassion and Not Now, Kiddo. Contrast with Stockholm Syndrome, Subverted Suspicion Aesop and Properly Paranoid, all of which happening when the hero is right all along. Related to Cassandra Truth, I Reject Your Reality, Moving the Goalposts, Not Now, We're Too Busy Crying over You and Windmill Crusader.
- The Rising of the Shield Hero:
- In a victory celebration immediately following the heroes stopping the second wave of calamity that threatened the world, Malty and King Melromarc inform the Spear Hero, Motoyasu, that the Shield Hero, Naofumi, has a slave girl named Raphtalia. Motoyasu demands a duel to free her, which the King forces Naofumi to accept. When Raphtalia tries to explain that she doesn't mind being Naofumi's slave and that he treats her nice, Motoyasu assumes that the slave brand must be forcing her to be loyal and the King orders Raphtalia to be bound and gagged so that she can't protest. Naofumi loses the duel due to Malty's illegal interference, but the moment Raphtalia is freed from the slave brand, she slaps Motoyasu and rushes to Naofumi's side. Everyone quickly comes up with the excuse that she was branded so long that her will was permanently destroyed by the slave brand, and when Naofumi later gets another slave (Firo), Motoyasu tries to pull the same trick only for Firo to give him a megaton kick to the crotch.
- The Church of the Three Heroes tries having Naofumi branded as a wanted criminal for kidnapping the Princess Melty. When Princess Melty tries explaining to the three heroes that she had left with him of her own free will, the Church and her sister Princess Malty tries rationalizing this as being the product of an ability to brainwash people that they believe the Shield Hero possesses. This is quickly shot down by Naofumi, citing that the other three heroes do not possess such an ability and that it would be very unlikely that he would have it. This also can be noted as further evidence that Malty lied about him raping her at the beginning of the series, as having the power to control people's minds would have been useful if he did do so.
- In Rainbow Rowell's Runaways, the team tries to "rescue" Klara from foster care, only to discover that she's been Happily Adopted. Unwilling to accept that Klara has good reasons for not wanting to return to the team, Gert suggests that maybe Klara's foster parents are supervillains who brainwashed her, as Molly's grandmother tried to do. Considering that Klara Has Two Daddies, this does not go over well, and Klara asks them to leave.
- Freeze On The Stones: In the Enchanted Forest, Belle and Rumpelstiltskin message her father Maurice that they have gotten married. When she visits him, he is relieved that she had "escaped" him. When she clarifies that she did not escape the castle and that the letter she sent was all true, Maurice is convinced that Rumpelstiltskin placed her under a spell to make such claims and that a quick trip to the clerics will clear everything up. During the curse in Storybrooke, Maurice's cursed self Moe French becomes worried and paranoid over his daughter Lacey's safety when she and her daughter Renee stay over at Mr. Gold's place under the assumption that he is physically and sexually abusing her. No matter how many times she tells him otherwise, he presumes that the truth is much worse than she claims under the assumption that Gold had threatened her if she told anybody.
- Near the end of Harry Potter and the Prisoner of Azkaban, Snape insists that Harry and his friends must have been bewitched to believe that Sirius Black was innocent. He rationalizes this as saving them from getting in trouble for attacking him, but it's clear that his grudge against Black for being one of his school bullies (and because he blames him for the death of Lily Potter) is the real reason.
- In Kamen Rider Gaim, Mitsuzane slowly undergoes Sanity Slippage and starts to see Kouta as a idiot who will doom the world with his bumbling instead of a selfless hero (largely out of resentment, as Kouta regularly interferes with his own "world-saving" plans and gets praised for it). When their mutual friend Mai tries to defend Kouta's actions, Mitsuzane ignores her argument and dismisses her as "confused", and starts threatening more of their friends to make her come with him. Most disturbingly, he doesn't even get mad or make any special effort to convince her she's wrong; he just takes the stance of "I'm sure you'll eventually see that I'm right."
- Defied in Undertale. When the protagonist first meets the two royal guards loyal to Undyne, Undyne will let them know that she had told them beforehand that she'd likely to have been brainwashed if she ever became friends with a human if they contact her after befriending her. This is why she doesn't bother coming to talk them into letting you through.
- Code Name Kids Next Door has Professor XXXL lure Nigel to his lair so he could use Nigel as a taste tester to develop the perfect snow cone in "Operation C.O.L.L.E.G.E." Despite the professor's relatively harmless motives, Nigel appears to be strapped to a chair while being fed snow cone after snow cone, which the KND mistakes for torture even though Nigel is enjoying it. When he's "rescued", his attempts to explain that he was fine and that it was all a big misunderstanding fall on deaf ears, as they believe he's suffering from Stockholm Syndrome.