Season 5, Episode 3 - The Pulled Groin Extrapolation
Aired September 29, 2011
When Amy needs a guest to attend a wedding with her, she knows better than to ask Sheldon and instead asks Leonard. The pair get to know each other when Leonard takes her to the wedding, while Sheldon gets obsessed with HO trains. Meanwhile, Howard and Bernadette spend their weekend with Mrs. Wolowitz.