When Leonard and Sheldon encounter a young woman crying on the curb, Leonard invites her to dinner. She is abrasive, sexy, hard, and sarcastic, but she softens a little when she discovers that the guys aren't trying to get something from her.

Brilliant physicist roommates Leonard and Sheldon meet their new neighbor Penny, who begins showing them that as much as they know about science, they know little about actual living.

Penny wants to throw a surprise party for Leonard, but Sheldon's indulgence at an electronics store throws her off-track.

Penny tags along on Sheldon's first ever date; Wolowitz finds a new use for a robotic arm.

Sheldon becomes agitated when his regular barber is out sick. Astronaut training causes Howard to reconsider wanting to go into space. Penny beats Leonard in chess, the first time she plays.

Howard and Bernadette welcome their new baby and Sheldon and Amy celebrate Amy’s birthday.