Phoebe has been chosen to do a slam dunk for the whole school. But how can she, not being tall enough? She reasons that maybe if she just knew how it felt, then maybe she could do it. Therefore, Ms. Frizzle takes the class into outer space and transforms the school bus into a planet with adjustable gravity. With little gravity, Phoebe finds a slam dunk easy, but with no gravity it is impossible to even make the ball go through the basket. However, when the lever accidentally falls into the position of heavy gravity, the class has trouble even standing up! They will have to restore the gravity to normal so they can return to Earth in time for Phoebe's performance.