The Plimpton Stimulation

"The Plimpton Stimulation" is the twenty-first episode of the third season of The Big Bang Theory. This episode first aired on May 10, 2010.

Summary
Dr. Elizabeth Plimpton visits at Sheldon's invitation and she seems more interested in with everyone rather than visiting Caltech.

Extended Plot
Dr. Elizabeth Plimpton, a from Princeton University, comes to Caltech for a visit. At first she seems like a typical absent minded professor. Sheldon arranges her trip, and offers her to stay in his bed (with Sheldon sleeping on the ), rather than a which he perceives as less hospitable. Leonard tries to impress her by talking to her about her work, but she's more interested in having sex with Leonard, which he very much welcomes. The next morning, Sheldon is completely oblivious to what happened overnight, but Penny immediately detects what happened based on Leonard's and Elizabeth's, and becomes , because she broke up with Leonard not too long ago. Later that day, at the Caltech cafeteria, Dr. Plimpton meets Raj and wants to bed him, too. That night, Howard and Leonard come to Raj's apartment for Halo night and find Elizabeth there, so she suggests sex with all three guys. Howard and Leonard are uncomfortable at this since it would involve seeing each other naked. Leonard ia also confused because he thought that she and Elizabeth shared something special. At last, Raj manages to lock the other two out and has sex with Elizabeth alone. At the end of the show, Leonard and Penny discuss whether or not he needs to explain to Penny what happened. Although Penny keeps insisting that he doesn't need to explain anything, Leonard explains that he had sex with Dr. Plimpton because "she let him".

Critics
"You can't argue with good farce. Even in a short scene like the one we got at the end here it can generate tremendous easy laughter. Dr Plimpton tries to seduce Raj, Howard and Leonard at the same time leading to a really funny punch line...The problem was the writers couldn't decide who the story was really about. The first half of the episode was all about Sheldon as the writers exploited every amusing thing he could say about a female guest coming to stay. Then once Leonard slept with Dr Plimpton the episode became about him and how Penny would be affected by his dalliance...A fun episode with plenty to keep you chuckling. But wait for it...this could have been so much more." - The TV Critic's Review

Quotes
Elizabeth Plimpton: Do you like ?

Howard: Yeah, sure. In fact, I'm a.

Elizabeth: Not tonight. Tonight, you are a delivery man. You brought soup, but, uh-oh, Raj and I don't have enough money to pay you. So we'll have to come to some other kind of arrangement.

Howard: Beg your pardon?

Elizabeth: You two figure out the details, I'm going to change into something I don't mind getting ripped off my milky flesh.

Howard: [after Elizabeth goes into another room] What the frak?

Raj: Go away. She wants New Delhi, not kosher deli. Besides, you have a girlfriend.

Howard: We broke up weeks ago.

Raj: Why didn't you say anything?

Howard: I was waiting for the right time. This is the right time.

Leonard: [arriving] Hey, who's ready for Halo?

Raj: Oh, this is like a nightmare. Get lost!

Howard: He's right. The numbers are shaky enough as it is.

Leonard: I don't understand.

Elizabeth: [peeking head through door] Oh, good. Leonard's here.

Raj: Good?!

Leonard: Elizabeth, what's going on?

Elizabeth: What's going on is you and Howard are my moving men and Raj is my new landlord and I don't have enough money to pay any of you. [goes back inside the room]

Leonard: Is she suggesting what I think she's suggesting?

Howard: Yep! Welcome to the Penthouse Forum.

Trivia

 * Sheldon states, "Pets are banned under The Roommate Agreement, with the exception of service animals, such as  and, one day, y-enhanced helper ."' In "The Zazzy Substitution", "The Luminous Fish Effect " and '"'The Ornithophobia Diffusion'", Sheldon violates this rule.
 * The yogurt is referred to again in Season 5, "The Weekend Vortex".