Leonard gets an offer to work on a Stephen Hawking project, but it means he will have to leave for 3-4 months. Sheldon tries to talk him out of it, but Leonard doesn't listen to him. When he tells Penny about the trip, she's extremely supportive. Before he leaves, his friends throw a party for him. 

Raj wants Lucy to meet his friends, but she thinks it would be overwhelming. After much discussion, Amy is selected as the first friend to meet her. The dinner doesn't go well when Raj pressures Lucy into defining if they are boyfriend/girlfriend. They make up and Raj invites her to the party for Leonard. Instead of showing up, Lucy breaks up with him by text. On the way home from the airport, Penny stops by to check on Raj. He's upset and believes he's unlovable. Penny says it's just the booze talking and Raj tells her he hasn't had a drink. He's cured! The episode ends with him talking non-stop to Penny, Amy, and Bernadette.