Leonard's mother, Mrs. Hofstadter (Christine Branksi) comes in to town to visit and the gang gets to meet her for the first time.  Mrs. Hofstadter is a psychiatrist when she meets Penny coming up the stairs, she has Penny in tears by the time they reach Leonard's.  We quickly learn that Mrs. Hofstadter and Sheldon are very similiar and the two quickly bond and we're even tricked into believing the two are about to get it on (when they end up just playing Rock Band).

Penny and Leonard, both depressed from what Leonard's mom (she told Penny that she has father issues and she never gives Leonard any affection), end up spending the night drinking tequila together.  They end up making out and when they go to Penny's bed, Leonard manages to blow it by making some inappropriate pillow talk (saying that they're both having sex with each other as a way of having sex with their parents!).  She kicks him out and they choose not to talk about it.

Also, Raj and Wolowitz meet Mrs. Hofstadter at work and she exposes them as having a ersatz homosexual marriage.  Very funny.