Dr. Beverly Crusher welcomes aboard an old mentor who tells her about aging and how he misses his wife who died of illness. She sees him to his quarters and goes to see Wesley in Engineering.
Wesley is working on a warp bubble experiment and causes an energy glitch. He looks up to find his mother no longer standing there.
Then the doctor is going to see her old mentor Dr. Quaice. She sees he is not in his quarters. When she asks the computer for his location, the computer states that there is on one by that name on board. She questions the crew about his whereabouts, even Chief O'Brien, who beamed him aboard. No one knows about his visit to the Enterprise. Data scans computer records to find no record of a Dr. Quaince anywhere.
As the doctor continues her inquiry, she discovers from the crew that there are only approximately 200 people on board, 800 less that originally. Slowly, more and more people go missing until it is only she and the captain left. Beverly tries to get him to remember his crew, including her son Wesley, who went missing as she was talking to him about the experiment he was working on. The captain finds nothing unusual about the two of them being the only ones on board exploring the galaxy.
As they sit on the bridge, the captain disappears as well, leaving her alone on the ship. Through this, she encounters atmospheric disturbances at random places on the ship, including on the bridge where she is almost sucked through a vortex.
Meanwhile; Wesley, Picard, Geordi, and the traveler (who first appeared in "Where No One Has Gone Before") is back to help get Beverly through the warp bubble before it collapses. The traveler explains that Beverly, when she got trapped in the bubble, may be living out the reality she created for herself when she first entered the bubble.
Back in the bubble, Beverly is realizing it is she who is trapped in this alternate universe and it's slowly collapsing. She connects the dots as it were---remembering Wesley's experiment with warp fields and what she was thinking at the time of the glitch. She asks the computer what is the nature of the universe and it replies that it's' a field of energy surrounding the ship. She knows she must get back on the other side fast.
Wesley, with the help of the travelor, are recreating the experiment and both phase out as Beverly finds another atmospheric disturbance and runs through. She is relieved to see Wesley and asks the captain how many people are onboard ship. He responds 1014, including her guest Dr. Quaice.
This episode was exciting on two levels---watching Wesley trying to get his mother back and Watching Beverly trying to get back to her real universe.