Updated: August 20, 2024

It is October 16, 1997 and the United States is proceeding towards the launch of one of history's great adventures: man's colonization of deep space. The Jupiter-2 ( called Gemini 12 in the pilot episode ), a futuristic saucer-shaped spaceship, stands on its launch pad undergoing final preparations. Its mission is to take a single family on a five and a half year journey ( stated as 98 years in the pilot episode ) to a planet of the nearby star Alpha Centauri ( the pilot episode refers to the planet itself as Alpha Centauri ), which space probes reveal possesses ideal conditions for human life. The Robinson family was selected from among 2 million volunteers for this mission. The family includes Professor John Robinson ( Guy Williams ), his wife, Maureen ( June Lockhart ), their children, Judy ( Marta Kristen ), Penny ( Angela Cartwright ), and Will ( Billy Mumy ). They will be accompanied by their pilot, US Space Corp Major Donald West ( Mark Goddard ), who is trained to fly the ship in the unlikely event that its sophisticated automatic guidance system malfunctions.

Other nations are racing the United States in the effort to colonize space, and they would stop at nothing, even sabotage, to stop the US effort. Dr. Zachary Smith ( Jonathan Harris ), a medical doctor and environmental control expert, is actually a foreign secret agent. He reprograms the Jupiter-2's B-9 environmental control robot ( voiced by Dick Tufeld ) to destroy critical systems on the spaceship eight hours after launch. Smith is trapped aboard at launch and his extra weight throws the Jupiter-2 off course, causing it to encounter a meteor storm. The robot's rampage causes the ship to become lost.

The Robinsons are often placed in danger by Smith, whose self-centered actions and laziness endanger the family. In the second and third seasons, Smith's role assumes a less evil overtone although he continues to display many character defects. In "The Time Merchant", Smith travels back in time to the day of the Jupiter-2 launch, with hope of changing his fate. He learns that without his weight altering the ship's course, it would be destroyed by an uncharted asteroid. In an act of redemption, Smith elects to re-board the ship, thus saving the Robinsons' lives.