: A major arc involves Amy and Sheldon moving in together following a flood in Amy's apartment. This transition forces Sheldon to confront his father's history and explains "lore" like his three-knock rule .
The season explores how the characters handle maturity and changing living arrangements:
: Howard and Bernadette welcome their daughter, Halley, in the mid-season episode " The Birthday Synchronicity ".
: Sheldon, Leonard, and Howard struggle under a deadline from the U.S. Air Force for their quantum gyroscope invention.