The Maternal Congruence
The Big Bang Theory Season 3 Episode 11 (12/14/09)
Leonard's mother, Beverly (Christine Baranski) is coming back in to town, and Penny is upset that Leonard neither told her nor even told his mom about them. Leonard goes to the airport to pick up his mother with Sheldon, only to find out the two of them have been keeping in contact. Leonard gets upset when he finds out that Sheldon already knew his mom was getting divorced, his brother got engaged, and his childhood dog died.
Leonard heads to his room upset, while Penny goes to drive Beverly to her hotel. On the ride, Penny gets annoyed so they stop for some drinks. As Beverly loosenes up, Penny admits she's been sleeping with Leonard, and Beverly seems to almost approve. She wants to take a bus boy home and ends up back at Leonard's (via cab luckily) where she gives Leonard the hug he wants. She sends him back into his room and kisses Sheldon, but decides she'd rather have the bus boy.
The episode ends with Sheldon, Penny and Leonard driving Beverly, very hungover, back to the airport and they decide not to tell Leonard about the kiss.