Arnold's cousin, Janet, visits Ms. Frizzle's class to go on a field trip to the planetarium to watch a show about the solar system. When the planetarium is closed, Arnold suggests they go into outer space. The class explores Mercury, Venus, and Mars, while Janet collects proof of their voyage. In the asteroid belt, while Janet is trying to collect an asteroid as further proof, they lose Ms. Frizzle and find themselves lost in space.
