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."

Banished from camp for his latest endangering behavior, Dr. Smith discovers alien plants that can duplicate things (including people), which he uses to his advantage.