While traveling through the Zeta star system the Jupiter 2 develops a problem with its energizing module, requiring Don and Prof. Robinson to find a planet to set down on for repairs. A space capsule of unknown origin locks onto the ship when the Jupiter's course intersects its orbit and the capsule follows the Jupiter 2 down to the third planet. Unknown to the Robinsons the capsule - identified as capsule Alpha 784 - had been placed in isolation orbit by Zetan Galaxy Law Enforcement and that it contains dangerous cargo.