On the Robinson's flight away from the plant, the Jupiter 2 is suddenly being targeted by a hyper atomic missile launched by the recently departed planet in protest to their leaving. They are then forced to alter their course immediately or else they will be destroyed.
After initially moving out of the missiles path, the danger is not over as it altered its course due to a homing device. The missile compensated for any evasive action so John orders Don to wait until the missile is about to hit the ship before using the main thrusters to move out of its way. They successfully achieve this but as they were so close to the planet the missile made the planet explode. The Robinsons are then caught in the gravitational pull of the planet so John orders Don to use all the power they have to get away. Don refuses to as the navigational system has been damaged but Dr. Smith, wanting to get away, attempts to apply all power however as a result they suffer a crash landing. When they land the ship is a mess and the conditions outside are foggy. Smith orders the Robot outside to test the conditions even though John warned him not to because the conditions could corrode his metal. While outside the Robot goes missing and when John finds out Smith ordered the Robot outside he forces him outside to locate it. While searching, Smith is overcome by fear at the sight of an alien creature and he collapses. After realising he too is missing, John and Don search for Smith but to no avail. After everyone had gone to sleep, Will takes it upon himself to search for Smith. When outside he too confronts the alien and is taken away by Captain Tiabo, the inhabitant on this mystery planet. Tiabo takes Will to his home where he meets up with Smith and the Robot. While there Tiabo contacts his superiors and Will, Smith and the Robot discover that Tiabo and his army have a weapon that they intend to unleash on them. This is because they thought the missile that was attacking the Robinsons was meant for Tiabo's planet. While at Tiabo's home Smith finds a most delicious drink but the Robot discovers that the drink is an explosive and Smith will detonate at any time. Knowing this, Tiabo lets them escape with the hope that Smith will explode in their spaceship. Desperate to find a neutraliser for Smith, Will makes his way back to Tiabo's home where he discovers that Tiabo has been pretending to contact his superiors the whole time and that he is alone on this planet. Tiabo admits to Will that he just wants to be alone and that he fears more people will continue to arrive on his planet. Will assured him they had no choice and are only here temporarily so Tiabo gives Will a neutralizer which cures Smith just in time.
All in all a very good episode, underrated by many and one of the strongest of the second season.