Everyone is confused and fascinated when a physics graduate student develops a strong attraction for Sheldon and becomes his assistant/platonic girlfriend. Sheldon is oblivious until she won't let him pursue his geeky interests.
Extended PlotA grad student Ramona Nowitzki, comes up to Sheldon at the cafeteria and is extremely enthused with his theoretical work. She gets herself an invitation to his apartment bringing Sheldon's favorite take-out food. After Ramona leaves from the cafeteria, Howard asks Sheldon if he knew what just happened. Sheldon replies yes, and that he is getting free take-out food. Later, at the apartment, the gang finds this turn of events fascinating watching from the couch until Ramona says that she thought she and Sheldon were going to be alone. Leonard gets everybody to leave though they beg to stay and watch this strange turn of events. Though it appears to be a date, a naive Sheldon just likes the idea of getting free food.
Ramona begins to help Sheldon with his work and other odd tasks such as doing his toenails. She keeps pushing Sheldon to concentrate all his spare time on his scientific endeavors, telling him that he needs to keep it up if he wants that Nobel Prize. Ramona is even spending the night at his apartment sleeping on the couch, keeping him in line. The loss of his freedom and leisure time (Halo, sleeping, comic books, and paintball) upsets Sheldon, but he can’t stop her or her obsession. He goes to see Penny because he realizes that he is in "some sort of relationship" with Ramona and Penny is so good at ending them.
Ramona is convinced that Penny is in love with Sheldon and that she has to give him up. Penny agrees to "give him up" and Ramona wants them to be sisters though she still has her eye on Penny. Finally, Sheldon gets rid of her when he makes his breakthrough and she asks if she could share credit on the paper calling it the Cooper-Nowitzki Theorem. He finally has the courage to tell her to get out. Penny sees her very upset heading down the stairs, and then she calls out to her asking if they were still sisters. Later, another student, Kathy O'Brian, is also enthused by Sheldon's work and visits the cafeteria to meet Sheldon and the gang like Ramona did, only now Kathy orders pizza for him. After Kathy leaves, Leonard asks Sheldon the same thing as before about him knowing what just happened. Sheldon replies the same answer last time, only now he says that he is getting a free pizza.
After leaving the apartment when they first met Ramona, Penny asks the others what "his deal" is (girls, guys, sock puppets?) and they generate a couple of possible scenarios of Sheldon reproducing using mitosis by splitting in two after eating too much Thai food or forming a chrysalis like a butterfly. She thanks them for the nightmares, though Leonard is the one who has the nightmare of Sheldon eating too much Thai food and splitting in two.
- Guest starring:
- Teleplay: Tim Doyle & Richard Rosenstock
- Story: Stephen Engel & Daley Haggar
- "Another well written and focused episode. It’s not particularly funny and I fear it is a worrying sign of certain things to come. But for now I give all concerned credit for creating something genuinely different to most other contemporary comedies." - The TV Critic's Review
- IMDb user reviews
- Title Reference: The title refers the theorem name suggested by Ramona at the end of the episode.
- This episode was watched by 9.67 million people with a rating of 3.8 (adults 18-49).
- Chuck Lorre's vanity card .
- Episode transcript 
- Sheldon doesn't eat at unknown restaurants, for fear they would try to pass off a "trident" as a fork (three tines, versus four).
- Sheldon and Leonard apparently have several clauses in their Roommate Agreement (here called a "Friendship Agreement", and the earliest mention of such a document) that they will help each other if one discovers (or causes) a world-ending event similar to those found in movies, including a Skynet clause, Body Snatchers clause, and Godzilla clause.
- Sheldon has a hammer toe that Ramona works on and freaks out Penny.
- Sheldon stumbles into a relationship.
- Both Ramona Nowitzki and Kathy O'Brien have red hair.
- Penny: Holy crap on a cracker! (twice)
- [Sheldon has just accepted a graduate student's request for dinner in his place]
- Sheldon Cooper: What a nice girl.
- Howard Wolowitz: Sheldon, do you have any idea what just happened?
- Sheldon Cooper: Yes, apparently I'm getting a free dinner.
- Penny: I know this is none of my business, but I just... I have to ask - what's Sheldon's deal?
- Leonard Hofstadter: What do you mean, "deal"?
- Penny: You know, like, what's his deal? Is it girls...? Guys...? Sock puppets...?
- Leonard Hofstadter: Honestly, we've been operating under the assumption that he has no deal.
- Penny: Come on, everyone has a deal.
- Howard Wolowitz: Not Sheldon. Over the years, we've formulated many theories about how he might reproduce. I'm an advocate of mitosis.
- Penny: I'm sorry?
- Howard Wolowitz: I believe one day Sheldon will eat an enormous amount of Thai food and split into two Sheldon's.
- Leonard Hofstadter: On the other hand, I think Sheldon might be the larval form of his species. Someday he'll spin a cocoon and emerge two months later with moth wings and an exoskeleton.
- Penny: Okay, well, thanks for the nightmares.
- Ramona Nowitzki: Oh, sorry I’m late. I just got so caught up reading the draft of your latest paper.
- Sheldon Cooper: Did you enjoy the humorous footnote where I illustrate mirror-symmetry by likening it to the Flash playing tennis with himself?
- Ramona Nowitzki: So funny. But the idea that you might be able to incorporate gravity, I have to tell you, I found it physically exhilarating.
- Sheldon Cooper: My hypotheses tend to have that effect.
- (After Ramona quotes Sheldon)
- Sheldon Cooper: (To Leonard) You heard her. How can I argue with myself?
- Penny: Don't go in there.
- Leonard Hofstadter: Why? What are they doing in there?
- Penny: I can only explain it in a therapist's office with dolls.
- (Penny goes in)
- Penny: Yeah, you're right.
- Sheldon Cooper: How can I ever repay you?
- Ramona Nowitzki: Would you consider naming it the Cooper-Nowitzki theorem?
- Sheldon Cooper: Who's Nowitzki?
- Ramona Nowitzki: I'm Nowitzki.
- Sheldon Cooper: You want me to share credit?
- Ramona Nowitzki: Uh-huh.
- Sheldon Cooper: Get out!
- (Outside in the hall, Penny is coming up the stairs just as Ramona comes out of the apartment with her bags)
- Penny: Oh, hey, hi,
- Ramona Nowitzki: (angrily) Oh, bite me! (She runs down the stairs)
- Penny: (confused) Sisters?