Cadets Astro, Roger Manning, and Eric Rattison, out on space maneuvers aboard the Polaris, run into trouble when the Servo unit (which controls the automatic steering unit) on the space ship goes out of commission, and they are forced to make an emergency landing on Jupiter. Problems with Jupiter's intense gravitation field and ammonia storms threaten the ships' survival, and also hinder the efforts of Tom and Captain Strong to come to their aid.