Season synopsis
Contains spoilers for this season and earlier seasons.
The four men return from their three-month Arctic expedition. Howard and Raj confess to manipulating Sheldon's research data, and he leaves for Texas to stay with Mary. The three follow him. After arguing with Mary and realizing he is uncomfortable at home, Sheldon agrees to return with them. After the expedition, Leonard and Penny reconnect and start dating, but their new routine exposes mismatched expectations: when Penny's former boyfriend Justin plans to stay with her, Leonard objects and Sheldon is distressed by their argument. They settle it after Justin agrees to sleep on Sheldon's couch.
Sheldon gets stuck on a physics problem and takes a menial job at the Cheesecake Factory with Penny to clear his mind. Leonard must choose whether to take Sheldon or Penny as his guest to CERN's Large Hadron Collider, making a roommate-agreement clause a real conflict. Howard and Bernadette, introduced by Penny, bond over meddling mothers; his early proposal does not settle their future. Sheldon's friendship with Penny deepens through her care for him and his awkward attempts to help her.
Leonard tells Penny he loves her, but she cannot say it back, and their relationship ends after Wil Wheaton's interference aggravates the strain. Sheldon struggles to spend time with both, and his spaghetti dinners help them resume speaking as friends. In the finale, Penny returns from a disappointing date and sleeps with Leonard, then says it was a mistake and that she does not want to restart the relationship. Meanwhile Howard and Raj secretly arrange a dating-site match for Sheldon. Amy Farrah Fowler appears only in this final episode; she and Sheldon agree to share a drink after Amy says she agreed with her mother to go on one date each year. Howard and Raj forced Sheldon to attend using a dirty sock.