A small, crab-like robot heralds the arrival of a family of colonists from an alien race called the Taurons. John believes that they are peaceful settlers, but Smith insists that they are the vanguard of an invading army.

Will's "message-in-a-bottle" experiments deplete the supply of fuel for the Jupiter 2. John and Don work on an alternate fuel source, but they cannot stabilize the output. Deciding to build a smaller, 2-man craft, they pull the reactor chamber out and use Will's experiments to construct a "space raft."