A visiting engineer's propulsion experiment throws the ship far across the galaxy.
Starfleet engineer Kosinski comes aboard to test a warp-engine modification, accompanied by a quiet alien assistant later known as the Traveler. The first test hurls the Enterprise into another galaxy, and a second sends it much farther, to a region where thoughts take physical shape and crew members see images from their pasts and fears.
It turns out that the Traveler, not Kosinski, is responsible for the leaps, and that he is now exhausted. With the crew focusing their thoughts on his well-being, he manages to bring the ship home before fading away.
The Traveler tells Picard that Wesley Crusher has exceptional potential, and Picard responds by making the boy an acting ensign. The Traveler returns twice in later seasons to guide Wesley.