The Infestation Hypothesis is the second episode of the fifth season of The Big Bang Theory.
Leonard is having a dinner date with Priya on Skype. Thus, Sheldon goes over to Penny’s place, where he discovers she has a new red armchair, which he finds comfortable. However, when Penny reveals that she got it off the curb, Sheldon freaks out.
At the university, Howard asks Leonard about his long-distance relationship with Priya, to which he says that they simply talk; Howard tells him that they’ve got to make love in some way.
That evening, Sheldon has a dream about Penny’s chair, in which he gets bugs crawling all over him, causing him to go over to Apartment 4B and convince Penny to get rid of the chair, but she refuses. He makes several attempts to persuade her to remove it, to no avail; she even uses the chair cushion to scare him off.
As Priya begins to undress in the middle of a video chat with Leonard, the screen suddenly freezes. The next day, Howard presents an invention used to simulate kissing, which he and Raj try out.
Sheldon meets with Amy as she is playing her harp and asks her to try and get Penny to dump her chair. Amy encourages Sheldon to kiss her in a place she hasn’t been before, making Sheldon assume she means a geographical location, leading to Amy asking if he wants to hear her playing the harp; doing so causes Sheldon to have a flashback.
Amy goes to Apartment 4B to speak with Penny regarding the chair. Penny quickly realizes that Sheldon put Amy up to it and tells her to leave. Amy apologizes and wants to remain friends with Penny, then sits in the chair to prove her loyalty. All of a sudden, something bites Amy in the butt, causing her to get up and see that there’s something moving inside the chair. Freaked out, Penny and Amy run out of the apartment screaming.
Once again on Skype with Priya, she tells Leonard that her parents are there, too, and they both look at him sternly. Howard and Raj discover the chair from Apartment 4B on the curb, and decide to bring it to Apartment 4A. As they carry the chair inside, whatever creature is in the chair is shown moving around the back of it.