The Cohabitation Formulation

The Cohabitation Formulation is the sixteenth episode of the fourth season of The Big Bang Theory.

Plot Summary
Priya is back and she decides to date Leonard  again. Howard is forced by Bernadette  to choose between him and his mother.

Extended Plot
Howard and Bernadette  just had sexual intercourse, after that Howard  immediately prepares to leave, Bernadette  feels cheapened by this and asks him to consider living together. Howard arrives home and as usual his overbearing mother; Mrs. Wolowitz annoys him with her rude comments, although this time Howard  snaps and leaves the house. Leonard opens the knocking door to find Howard  who request to stay at their apartment. Sheldon comes out and commands Leonard to make tea as Howard  is depressed, Sheldon  then tries to comfort Howard, but Howard  accidentally mentions that Priya  is in town and is staying with Raj. Because Leonard and Priya had a "past" he dashed toward Raj's  House, when he got there He and Priya  talked in private, although Raj  clearly forbade it. The gang is at the Cheesecake Factory when Leonard and Priya  announced that they will be seeing each other again, Bernadette  told Howard  to choose between her and his mother. Amy has heard the news from Sheldon  and rushed to Penny  side to comfort her. Howard shows up at Bernadette's  front door and explains that he chooses her and has moved out of his mother's  house. The gang is having dinner, Leonard and Priya  are acting all lovey-dovey and this annoys Raj. Penny joins in and talks to Priya, Amy  points out that Priya  is an amazing human. After having sexual intercourse, Howard ask her for all the request his mother  usually carries out, Bernadette  suddenly realizes how dependent the two of them are upon each other, Howard  moves back into his mother's  house. Amy decides to check up on Penny, when the two of them are talking Penny suddenly burst into tears, Amy  grasps this golden opportunity to examine her brain for data. Later, Penny comes into the apartment to eat Thai food with Sheldon  who is video-chatting with Amy. Amy tells Penny that thanks to her contribution she was able to make a monkey cry.

Trivia

 * Howard and Mrs. Wolowitz agrees that all CSI series has their pros.
 * Sheldon describes that he is Howard's  tertiary friend, Leonard  being secondary and Raj  being primary.
 * In this episode Sheldon shows surprising empathy.