When Wesley's experiment with a warp field goes awry, Dr. Crusher is unknowingly catapulted into a universe created by her own mind.

After escorting her friend Dr. Dalen Quaice aboard the U.S.S. Enterprise, Beverly is deeply moved by his sadness over losing his wife and other people he has loved. She seeks out her son Wesley, and finds him in engineering, working with a warp bubble on a computer screen. While she watches, his experiment inadvertently causes a strange flash of light. Soon afterward, Dr. Crusher is unable to locate Dr. Quaice. The ship's computer reports that he has never been on board, and Picard and Worf inform her that they had no knowledge of his presence on the ship. Additionally, O'Brien does not recall transporting Quaice aboard, and Starfleet has no record of the doctor's existence.

Beverly's bewilderment turns to concern when members of the crew, including her entire medical staff, begin to disappear. Neither their relatives, other crew members, or the ship's computer remember their ever having existed. Picard, Riker, and the crew are supportive and helpful, but since none of them remember the missing people, or feel like anything is wrong, Beverly wonders if she is suffering from delusions. She heads to Troi's office for analysis, and is confronted by a strange, swirling vortex which knocks her to the ground.

The crew discounts the possibility that Wesley's experiment caused the vortex. Beverly then asks if all crew members are accounted for. Data replies that no crew members are missing, and tells her there are 114 people aboard the starship.

Beverly panics, knowing the U.S.S. Enterprise population should be well over 1000. She requests that Worf monitor all remaining personnel, and is met with silence. No one has ever heard of Worf. She rushes to Wesley to try to link her predicament to his experiment, but Wesley soon disappears too. Beverly returns to the Bridge to find everyone but Picard vanished. Then the captain disappears, and the vortex reappears on the Bridge. While she struggles with it, Wesley works on the computer, despondent that he has lost his mother. At that moment, an alien called the Traveler phases in.

The Traveler explains that Wesley somehow summoned him, and tells the boy that at the moment the light flashed during his experiment with warp fields, Beverly was caught in a warp bubble and created her own reality based on what she was thinking at that precise moment — the tragedy of losing close friends. He begins working with the boy to try to recreate the bubble and bring his mother back. Meanwhile, Beverly uses the computer to determine that she is trapped in the bubble, and that it, along with her self-created universe, is collapsing. While the crew tries to maneuver the ship back to the exact spot where Wesley's bubble was created, Beverly runs to engineering, where she first saw the flash of light, as the ship begins to collapse around her. She locates the vortex with only seconds to spare, leaps through it, and suddenly appears back in the engineering room, surrounded by the crew and her very relieved son.