Season 1, Episode 6 - The Middle-Earth Paradigm
Aired October 29, 2007
The whole gang attends Penny's Halloween party, where Leonard's feelings for Penny come to the fore when her ex-boyfriend Kurt shows up. Leonard's attempts to impress Penny by standing up to Kurt lead to humiliation for Leonard.
Season 2, Episode 1 - The Bad Fish Paradigm
Aired September 22, 2008
When her first date with Leonard doesn't go to plan, Penny finds an unwilling confidant in Sheldon. Sheldon can not handle the stress of keeping Penny's secret, so he decides to move out of the apartment.