Big Bang Theory: The Alien Parasite Hypothesis

"The Alien Parasite Hypothesis" is the tenth episode of season four of the comedy series The Big Bang Theory and the nineteenth episode of the series overall. It was directed by Mark Cendrowski with a teleplay written by Bill Prady and Lee Aronsohn based on a story treatment by Chuck Lorre. It first aired on CBS on Monday, December 9th, 2010. In this episode, Leonard Hofstadter is looking to get back into the dating game. Discouraged that things never moved forward with Penny, Leonard calls upon the affection of his occasional sexual stress release partner, Leslie Winkle, which ultimately makes Penny jealous. And Sheldon Cooper complains about the historical inaccuracies of a local Renaissance Fair.