As the Robinson party travels through space in the Jupiter 2, Dr. Smith is seen taking care of a plant that he has brought with him from the last planet the Robinsons visited. Will warns Smith to throw the plant overboard. Before Smith can do this the Jupiter 2 passes through a radiation belt of some kind and the plant grows much larger.
Smith throws the plant overboard. It attaches itself to the ship and proceeds to foul up the normal operation of the Jupiter 2. The Professor and Major West try to fix this problem by entering the atmosphere of a planet that the Jupiter 2 is passing by, with the objective of using the heat from the friction of the Jupiter 2 entering the atmosphere to kill the plant. This appears to succeed, but an energy beam from the planet disables the Jupiter 2.
While the Jupiter 2 orbits the planet, West "recruits" Smith to come with him in the space pod down to the planet's surface to find and disable the source of the attack. Once on the planet, Smith and West find the vestiges of an ancient, almost extinct, warrior society called the Sobrams. The last living Sobram forces West to agree to remain on the planet to maintain the heritage of the warrior civilization.
Meanwhile, the space pod has automatically returned to the Jupiter 2, and Professor Robinson, the Robot, and Will use it to return to the planet. They find Smith and West, and the Robot discerns that West will not stay behind to maintain the planet, but will engage in a final battle with the remaining Sobram warrior. Smith gets the idea to replace West with the Robot in this task, and the Robot agrees.
While returning to the space pod, Professor Robinson finds the plant that Smith discarded earlier in the episode. He convinces the Sobram warrior to fight against the plant and let the Robot go. The warrior agrees, and the entire party returns to the Jupiter 2 safely.
- In one scene where the Space Pod is returning to the Jupiter 2, the sound of the Pod can be heard inside the Jupiter 2. This is not physically possible, as sound waves cannot travel through the vacuum of outer space.
- West states that the plant has clogged the Jupiter 2's intake ducts, but the ship does not have air breathing engines and any ventilation system that connects to the outside would be closed while the ship is in outer space. Perhaps the plant was growing over something that vented heat to space, which might be its only energy source in the vacuum and cold of space.
- In the absence of a gravitational field there is no need for stabilizers and unless the mass of the plant is on the same scale as the ship itself its motion on the outer hull will not produce any type of rotation. If this wasn't the case the Robinsons could start spinning the ship by just walking in a circle around the astrogator.
- How the plant manages to hold onto the smooth curved metallic surface of the ship as the Jupiter 2 dives through the planet's atmosphere and experiences terrific winds is not explained.
- The plant creature uses the energy of the Sobram weapons to duplicate itself. It also manages to duplicate its helmet and cache of neutron grenades.