Leonard returns from a summer at sea, and Penny's reaction sets the tone for the season. Sheldon adjusts to having his roommate back.
Leonard lies about something that happened while he was away and it threatens his relationship with Penny. Trust becomes the central issue.
A scavenger hunt designed by Raj pairs the gang in unexpected teams. The clues lead to revelations and bonding moments.
Amy helps Sheldon realize that his favorite childhood movie has a plot hole. His world is shaken.
Bernadette works at the same university as the guys, changing the dynamic. Sheldon worries about Amy being too close.
Howard writes a love song for Bernadette, and it turns into a viral hit. The musical spotlight suits him.
Professor Proton returns and Sheldon is jealous when he bonds with a young admirer. The mentor-mentee relationship gets complicated.
Sheldon can't stop thinking about an unsolved problem and it drives him (and everyone around him) crazy.
The gang shares Thanksgiving together and relationship milestones are discussed. Penny and Bernadette bond over in-law stories.
Sheldon accidentally makes a scientific breakthrough and has trouble accepting that it was a mistake. His ego takes a hit.
The group imagines what their lives would be like if Sheldon had never come to Pasadena. It's a Wonderful Life meets theoretical physics.
Penny gets a callback for a movie role and the excitement is real. Sheldon hesitates about moving forward with Amy.
Penny quits acting to pursue pharmaceutical sales. Leonard supports her while Sheldon questions the career change.
The guys try to get into Comic-Con and the lengths they'll go to are impressive. Sheldon's plan is characteristically elaborate.
Sheldon and Amy go on a train ride together and the experience sparks genuine romantic feelings. A milestone moment for them.
Leonard buys a dining table and Sheldon sees it as a threat to their couch-based lifestyle. The furniture debate gets philosophical.
Howard and Raj's friendship hits a rough patch. Bernadette mediates while Sheldon offers unhelpful advice.
Amy and Sheldon visit her mother and the awkward family dynamics give Sheldon a new appreciation for his own upbringing.
The guys face multiple choices and none of them can make a decision. Analysis paralysis hits the whole group.
Sheldon tries to move on from string theory and explores new scientific directions. The existential crisis is real.
The gang tries a spontaneous anything-can-happen night. Sheldon's attempt at being unpredictable is adorably predictable.
Professor Proton passes away and Sheldon processes his grief in a surprisingly touching way. The loss hits harder than expected.
Penny's acting career officially ends when she confronts her agent. Meanwhile, Raj and Emily take a step forward.
The season finale shakes up the status quo — Sheldon considers leaving Pasadena, Howard's mom has a health scare, and big decisions loom.