The Good Guy Fluctuation (2011)

A successful Halloween prank from the group enrages Sheldon, who promptly starts working on his revenge. His efforts all end up being fruitless, as Sheldon makes his pranks too obvious to the others by behaving oddly. Leonard reverses his prank on him, while Raj is delighted by the snake Sheldon puts into his desk drawer. He finally seems to have succeeded in tricking Howard with an electric handshake. When Howard breaks down because of a heart condition, Sheldon is terrified. Bernadette orders him to inject adrenaline into Howard's heart. After doing so, he is astonished to find out that it all was a ruse by Howard and Bernadette. Sheldon then ends up shocking himself while wondering how he could have fallen for that.

Meanwhile, Leonard meets a good-looking girl called Alice at the comic book store who also happens to be a comic book enthusiast. She gives him her phone number to discuss trading comic books and they end up kissing while at the apartment. Leonard finds himself in a dilemma, as he can either be a "good guy" and stay loyal to Priya or a "bad guy" and have an affair with Alice. A discussion with Penny doesn't help, and after Sheldon makes him feel less guilty by citing Friedrich Nietzsche, Leonard decides to go to Alice's apartment. While they are making out, he changes his mind and confesses that he has a girlfriend, prompting Alice to throw him out immediately. At the end of the episode, Leonard video calls Priya and tells her about Alice. Priya is seemingly okay with it, prompting Leonard to become suspicious. When he asks her about it, Priya reveals that she also cheated on him, as she had sex with her ex-boyfriend a couple of weeks ago. Leonard is upset and ends the call early. At that moment, Sheldon leaps out from under the couch cushions in zombie face paint, scaring Leonard. He triumphantly shouts "Bazinga, punk! Now we're even!".

Recurring characters: Aarti Mann as Priya Koothrappali, Kevin Sussman as Stuart

Title reference: Leonard torn between being loyal to Priya or having an affair with Alice. When confessing to Alice, Leonard calls 'being a good guy' his 'superpower'.