After playing tennis Elaine is recognized by Mrs. Landis (Gail Strickland), a former potential employer from Doubleday. Elaine lends Mr. Pitt’s tennis racket, a Bruline, to her in hopes of getting a new job. Meanwhile, Jerry is dating Sandy, a “non-laugher”. George is dating Nina (Charlotte Lewis), a model who eats a lot and who Kramer thinks may be bulimic.
George has dinner with Nina; she feasts on the food and then goes to the bathroom to “freshen” herself. Jerry has dinner with Sandy; he keeps making funny remarks but she doesn’t laugh, she just says “that’s funny”. Back at the apartment, Jerry mentions how the jokes just bounce off his girlfriend. George is now certain his girlfriend is bulimic because he heard a “blah” in the bathroom, and wants a matron to spy on Nina to verify if she intentionally throws up. We then meet Kramer’s mother, Babs (Sheree North), who happens to be a matron, but Kramer doesn’t want anything to do with her.
Mrs. Landis, who was injured playing tennis, is devastated and states she has no reason to live if she can’t play tennis, and Elaine doesn’t have the heart to ask for the racket back. George and Kramer both enter the restaurant Kramer’s mother works in, and Babs Kramer reveals her son’s first name: Cosmo. Back at the apartment, Elaine says Mr. Pitt needs his racket in a match against Ethel Kennedy. George comes with the news of Kramer’s name, much to everyone’s delight. Cosmo Kramer accepts his name and says he was running from his true self.
When Jerry goes to his girlfriend’s apartment he meets Laura, Sandy’s roommate, who does laugh at his jokes. Later, he talks with George, saying he intends to try the “switch”. George declares it is impossible: no one in Western history has ever switched a girlfriend for her roommate. Jerry and George spend long hours devising a plan to accomplish the switch. Finally, George comes up with the plot: Jerry will suggest a ménage à trois, Sandy will be disgusted, break up with Jerry, and tell Laura, who in turn will feel flattered, thus preparing the way for Jerry to date Laura.
While Mrs. Landis is out for lunch Elaine sneaks into her office to retrieve the racket. However, she is caught by Mrs. Landis’ assistant.
Kramer now uses his first name regularly. He tells his mother to quit her job as a matron. George’s plan with Nina goes wrong because of this, so he himself goes into the women’s bathroom to spy on his girlfriend. He hears the noise of someone being sick and shouts “Aha!” but it turns out to be another woman he hears and his date walks out on him.
As it turned out, Jerry’s girlfriend, Sandy, and her roommate, Laura, both accepted the Ménage à trois. George is ecstatic about it but Jerry doesn’t accept because he doesn’t want to become an “orgy guy”. Meanwhile, Newman, who is a good tennis player, is out of town so Kramer may use his keys to retrieve a Bruline tennis racket just like Mr. Pitt’s. When they open the door they see Babs in a compromising position with Newman. After she exclaims her son’s name in surprise, Newman, looking as surprised as ever, replies, “Cosmo?”