"The Spaghetti Catalyst" is the twentieth episode of the third season of the American sitcom The Big Bang Theory. This episode first aired on Monday, May 3, 2010.


Sheldon has a hard time trying to be friends with Penny after her breakup with Leonard. Later, Leonard and Penny decide to still be friends and are acting like divorced parents when it comes to Sheldon.

Extended Plot

The Spaghetti Catalyst - Sheldon and Penny meet in the hallway

With little pieces of hot dog?

The Spaghetti Catalyst - The group

Thank you, Penny!

Leonard and Penny are still fresh from their break-up, and it is time for the guys to "pick sides" about who will remain friends. Sheldon, as usual, is oblivious to this social convention and keeps contact with Penny, going as far as inviting himself over for spaghetti (with cut-up hot dogs reminiscent of his mother). When Sheldon reveals to the guys how he's been talking to Penny, Howard explains to him how he cannot maintain friendship with both her and Leonard.

In order not to arise suspicions, Sheldon eats at his apartment, then sneaks off to the refrigerator to grab a pack of hot dogs to take over to Penny's. Just as he's about to leave with the excuse of taking a walk, however, Howard leaves as well, insisting that the two go down together. Sheldon tries to shake off Howard, but ends up accompanying him right to his parked scooter. When he finally gets rid of his friend, Sheldon bumps into a dog which smells the hot dogs, and has to run for his life. He manages to shake the dog off and arrives at Penny's in a state of disgrace and holding a single hot dog. The two eat spaghetti together, and Sheldon ends up with a stomach ache from eating too much.

Sheldon has trouble keeping his friendship with Penny secret from his roommate, and ends up recounting his continued friendship to Leonard in the middle of the night. Leonard, however, seems not to care about this relationship. Sometime later, Penny and Leonard bump into each other in the laundry room, where the two try to keep things friendly between each other. Leonard explains how he's fine with Sheldon and Penny being friends, but seems upset when he learns that Penny is taking Sheldon for sheets, shoes and to a trip to Disneyland. Leonard gives Penny suggestions on how to take care of Sheldon at Disneyland, as he is familiar with the events of Sheldon eating junk food and throwing it up after he goes on Space Mountain, and is scared of Goofy, but alright with Pluto.

The remainder of this episode showcases Leonard and Penny as "separated parents" to their "son" Sheldon. Penny, it seems, has ignored Leonard's tips and Sheldon reacts exactly as Leonard had predicted. The two see Sheldon off to bed, and Penny admits to Leonard how they can remain friends despite their break-up. Leonard offhandedly tries to suggest they remain "friends with benefits". Sheldon is then having nightmares about Goofy after they leave.



  • "At least the issue of Penny and Leonard hanging out is fully addressed here. You can imagine a scenario where they would use Sheldon as an excuse to cover the awkward moments in conversation...Again the writers just have no interest in characterization and would rather have Sheldon camp his way through silly lie after silly lie. It's no more than should have been expected though, Penny and Leonard's relationship received no attention when they were together so why should it once they break up?...A one-dimensional episode from a show in danger of becoming one-dimensional." - The TV Critic's Review
  • IMDb user reviews


  • Title Reference: Sheldon's spaghetti dinner with Penny is the catalyst for repairing Leonard and Penny's friendship.
  • Chuck Lorre's vanity card [1]
  • This episode was watched by 11.63 million people with a rating of 4.6 (adults 18-49).
  • This episode aired in Canada on May 3, 2010 with 1.87 million viewers and a weekly ranking of #12.  
  • Episode transcript [2]

Costume Notes

Violet Lantern

BDG Toxic Stripes


With her waitress uniform, Penny wears her Truly Madly Deeply Celestial Printed Hoodie (discontinued by Urban Outfitters), and in the laundry room, she wears Lucky Brand Clinched Floral Crop Pants, and burgundy red Ugg boots. During dinner with Sheldon, she wears a grey Nebraska Cornhuskers t-shirt.



Sheldon: Go, Team Leonard!
(Holds a "L" on his forehead which typically stands for "loser")

Sheldon: Well, as my Meemaw would say, looks like we butchered a pig, but nobody wanted bacon. Oh and as for the tangent line; Sheldon and the hell hound, OR, How I lost my hot dogs.

Penny: He’s such an angel when he’s asleep.
Leonard: Yeah. Shame he has to wake up.
Penny: I think we can do it.
Leonard: Smother Sheldon in his sleep? Wouldn't that be wrong?
Penny: No, be friends. You and me.
Leonard: Oh. Sure. Absolutely.
Penny: Good. I’m glad.
Leonard: Here’s an idea. I’m just throwing it out there, friends who have sex.
Penny: Good night, Leonard.

Sheldon: (knock, knock, knock) Penny, (knock, knock, knock) Penny. (Long pause, knock, knock, knock) Penny. (Penny opens door, and a disheveled Sheldon is holding up one hot dog.) Here. I had to trade the others for my life.

Sheldon: (in his sleep) No, Goofy, no...