The U.S.S. Enterprise and crew travel to uncharted territory when a test on the ship's propulsion system backfires and blasts the starship more than a billion light years from their own galaxy.

The blame for the mishap is originally placed on Kosinski, an arrogant Starfleet propulsion expert. However, it is eventually discovered that the mental powers of the Traveler, a meek, humanoid alien (acting as the propulsion expert's assistant), was the true catalyst for this potential catastrophe.

Complicating the dilemma of being lost in space, the physical and mental world are integrated in this galaxy — what the crew thinks becomes real. After seeing his crew as well as himself experience alternate realities, the Captain orders the passengers of the starship to control their thoughts.

To make matters worse, their only hope of returning to their own galaxy lies in the mental abilities of the alien, who is exhausted and dying. Captain Picard and crew soon discover they must focus their thoughts on the assistant's recovery in order to discover a way back to their galaxy.