As has become the custom on many season finales, the characters are given something to do in story time corresponding with the three months of real time between seasons. Taking the chance to go down in history as the provers of string theory, Leonard, Howard, and Raj agree to go on a three-month expedition helping Sheldon search for monopoles at the magnetic North Pole.
The news surprises Penny, and Leonard starts to wonder if he should go or stay. Penny presents him with a Snuggie and gives him an extra-long hug. He asks her what these things mean, and she downplays them as friendly gestures. Only after they say goodbye does she admit to herself she doesn't want him to go.
A familiar scenario made fresh by the characters and the actors playing it.