In the year 1997, Earth is suffering from massive overpopulation. Professor John Robinson, his wife Maureen, their children (Judy, Penny and Will) and Major Don West are selected to go to the third planet in the Alpha Centauri star system to establish a colony so that other Earth people can settle there. They are to go there on a ship christened the Jupiter 2. However, Doctor Zachary Smith, an agent for an enemy government, is sent to sabotage the mission. He is successful in reprogramming the ship's robot, but in the process becomes trapped on the ship, and because of his excess weight, the ship and all on board become hopelessly lost and it now becomes a fight for survival as the crew tries to find their way back home.