The 13th episode of Season 5 of The Big Bang Theory , titled was the series' milestone 100th episode . It originally aired on January 19, 2012, and features a unique "All Just a Dream" narrative structure . Episode Summary
You can stream The Big Bang Theory Season 5, Episode 13 on the following platforms: The Recombination Hypothesis | The Big Bang Theory Wiki
About 90% of the episode is revealed to be an extended daydream in Leonard's head as he overthinks how the date might go . In this vision, their dinner is disastrous, leading to a major argument and then a secret physical relationship to avoid their friends' judgment .
After seeing Penny in the hallway, Leonard impulsively asks her out for a romantic dinner, which she accepts .