Leonard met Stephanie Barnett in "The Lizard-Spock Expansion" through Howard, who met her in a club. Howard told her she can drive the Mars Rover if she would join him. Eventually, that plan backfired. After Stephanie's last appearance in "The Vartabedian Conundrum", it seems they have broken up when he asked her to move out without any further explanation. Good chemistry, but too fast.
- "The Lizard-Spock Expansion" - Leonard met Stephanie because he and the boys were called by Howard to help with the Mars Rover incident. Howard introduced them to Stephanie who was a woman he was trying impress. After they escaped Howard's sight, Stephanie passionately kisses Leonard again and again. Later that episode, Leonard met Stephanie again and has intercourse with her. Out of guilt, Leonard tried to inform Howard, but Stephanie managed to inform him first. This made Howard angry at both of them, but was able to forgive them when Stephanie set him up with her roommate Lisa.
- "The White Asparagus Triangulation" - Sheldon finds that Stephanie is an important part of their social group butts into the Leonard and Stephanie relationship as a third wheel. She is the only girlfriend of Leonard's that he ever approved of especially since she is a doctor and gets to keep checking his hypochondriac symptoms. Though it seems Sheldon failed, but he was able to get Stephanie to change her Facebook relationship status from "single" to "in a relationship with Leonard Hofstadter."
- "The Vartabedian Conundrum" - Leonard realized that he is living together with Stephanie after Penny points out all of her possessions that are in his bedroom and his Bat Signal is missing. Leonard feels they have been moving too fast and wants to slow things down, but always fails to express it because she always cries, and then they have sex. So Leonard decides to ask her to move out via text.