The fallout from Raj and Penny's night together gets sorted out. Leonard processes the news. Sheldon and Amy's relationship takes a formal step.
Penny's apartment gets bedbugs and she crashes with the guys. Sheldon's germaphobia kicks into high gear.
Amy takes Leonard as a plus-one to a wedding and they bond over being overlooked by their respective partners.
Raj falls for the woman at the counter of his regular restaurant. Sheldon gets hooked on mobile gaming with unexpected intensity.
Howard gets a chance to go to space and the group reacts with a mix of excitement and concern. The mission prep begins.
Sheldon's mom Mary visits and the dynamic with Amy gets interesting. Howard deals with his mother's reaction to the space mission.
Leonard is tempted to cheat when an attractive woman shows interest in him at a Halloween party. His moral compass gets tested.
Amy feels left out when Bernadette and Penny go bridesmaid dress shopping without her. The hurt feelings are genuine.
Sheldon faces his fear of birds when one gets into the apartment. Penny and Amy push Leonard about his relationship avoidance.
Amy and Sheldon negotiate the terms of their relationship, landing on a unique arrangement. The flaming spittoon scene is classic.
Leonard's high school bully reappears and wants to apologize. Facing the past proves harder than expected.
Sheldon discovers magic tricks can dazzle Amy, giving him a new way to express affection. The shiny trinket approach is very Sheldon.
Leonard and Penny get back together in a hypothetical scenario that turns real. The on-again, off-again cycle takes a hopeful turn.
Leonard and Penny test their renewed relationship with a series of couple's challenges. They're keeping score.
Sheldon is hurt when Leonard wants to change the roommate agreement. The friendship contract gets a serious review.
Sheldon has to take a vacation and has no idea how to relax. Amy suggests working in her lab, and the results are oddly productive.
Kripke wants Sheldon's office and the university politics get cutthroat. Sheldon fights for his territory with characteristic intensity.
Sheldon disrupts his schedule and tries going with the flow. The gang barely recognizes him. Barbers everywhere breathe a sigh of relief.
The guys plan a gaming weekend and the girlfriends push back. The battle between boy time and couple time is eternal.
Sheldon struggles with a moral dilemma involving a Star Trek toy. Leonard's guilt about something from the past resurfaces.
Howard needs Sheldon to provide a recommendation for his space mission and Sheldon leverages the situation. Stephen Hawking makes a memorable appearance.
Howard's bachelor party doesn't go as planned. The best man duties fall to a surprising person, and the pre-wedding chaos is real.
Howard and Bernadette's wedding approaches and the launch date for his space mission coincides. Leonard and Penny make a big decision.
The season finale — Howard launches into space, the gang watches together, and the countdown to liftoff is equal parts terrifying and thrilling.