Leonard and Amy take a road trip.

Howard takes a class that Sheldon's teaching.

Howard is invited by NASA to throw the first pitch at a game.

Penny wonders why Raj's girlfriend doesn't like her.

The guys unsuccessfully try to invent "the next big thing".

Sheldon and Raj venture underground in the name of science.

Penny's flirtatious sales techniques work a little too well.

Love is in the air when the gang recreates a prom night.

Leonard's minor surgery leads to Sheldon's major reaction.

Sheldon records the final instalment of "Fun with Flags".

Amy forces Sheldon to celebrate a Victorian Christmas.

The guys finally agree to do something the girls like.

Sheldon asks his friends to challenge him.

Leonard and Sheldon defend themselves from an online bully.

Everyone awaits the reopening of Stuart's comic bookstore.

Sheldon and Penny try an experiment in falling in love.

Sheldon applies for a one-way mission to colonise Mars.

A final family dinner is held for Mrs. Wolowitz.

Sheldon and Leonard try to sneak onto Skywalker Ranch.

Wolowitz meets the half-brother he never knew he had.

Penny contemplates a return to acting.

Raj's father cuts him off from the family fortune.

Sparks fly when Leonard and Sheldon's mothers finally meet.

Sheldon pushes Leonard and Penny to set a date.