In 1997, as planet Earth becomes critically overpopulated, the Robinson family is selected from more than two million volunteers to begin the conquest of space. Believing that planets revolving around the nearby star Alpha Centauri may have atmospheres similar to that of Earth. Scientists construct the Jupiter II, the most sophisticated rocket ship yet built.Determined to beat the United States to Alpha Centauri, an enemy nation (presumably the U.S.S.R.) resorts to sabotage. Sneaking aboard the Jupiter II, Colonel Zachary Smith programs the environmental control robot to destroy the ship eight hours after take-off.
As countdown preparations continue, the Robinson family -- collectively representing a unique balance of scientific knowledge, emotional stability, and pioneer resourcefulness -- board the craft. As they enter the freezing chambers, where they are to spend the five-and-a-half-year voyage in suspended animation, all hatches are secured and Smith is trapped inside the craft.
Shortly after Jupiter II is launched, the craft penetrates a meteor shower and, because of Smith's extra weight, is unable to escape damage. Unable to save the ship himself, Smith enters the freezing chamber and releases the pilot, Major Donald West. Unaware that Smith is an enemy, West revives the Robinson family: John, an astrophysicist; his wife Maureen, a biochemist; and their children Judy, Penny, and Will.
As West struggles to maneuver the ship, Smith attempts but fails to re-program the robot. Totally thrown off course and unable to maintain orbit, the ship crash-lands on an unknown planet, which becomes a temporary home and enables them to repair their ship.
The series depicts their adventures in space and John Robinson's attempts to continue with the original goal and find Alpha Centauri, in defiance of Dr. Smith's endless attempts to thwart his efforts and return to Earth.