The Cohabitation Formulation
Bernadette is upset because Howard always goes home after sex so that he can help his mother in the morning. She insists that Howard moves in with her, which Howard is not interested in until he has a big fight with his mother after reaching home regarding his sexual activity with Bernadette. After spending the rest of the night in Leonard and Sheldon’s apartment, Howard moves out of his house to Bernadette’s apartment. Both are happy initially, but later Bernadette realises that Howard is too dependent on his mother and insists that he should not expect the same treatment from her. As a result, Howard returns home, where his mother (who did not know that Howard had moved out) grounds him for “running away”. Meanwhile, after Howard offhandedly mentions that Raj’s younger sister Priya has come to Pasadena and is staying at Raj’s apartment, Leonard rushes over there to talk to her. Leonard and Priya then decide to pursue a long-term relationship much to Raj’s indignance, who even quotes from the Manusmriti to dissaude Priya from dating Leonard, only for Priya to remind him that he eats beef which is also forbidden in the Manusmriti. Amy, who considers Penny her BFF, rushes over to Penny’s apartment to console her when she hears the news, but Penny insists that she is not upset that Leonard and Priya are dating. Later while having dinner, Penny finds out that Priya, being a lawyer, is much more successful than her and also sees that Leonard (and even Amy) is crazy about Priya’s achievements. After returning to her apartment following dinner, Penny breaks down crying, which confirms that she still has feelings for Leonard. Amy, who is also in Penny’s apartment, comforts her before using electrodes to determine which brain regions are active during crying, as she plans to make a rhesus monkey cry by stimulating the analogous areas of its brain. During the weekend, with Leonard and Priya having gone to Santa Catalina Island, Howard grounded by his mother and Raj at home in strong disapproval of Leonard and Priya dating, Penny spends her time with Sheldon and Amy (via webcam), who tells her that thanks to her contribution, she was successfully able to make a rhesus monkey cry.
Title reference: Howard deciding to live with Bernadette.