Geordi comes up with an idea to help the crew map an extremely dense globular cluster more efficiently. With Riker's permission, he channels the ship's warp energy into the main deflector grid, and the idea appears to work beautifully. After a few minutes, however, the plan appears to go terribly wrong when sensors indicate an explosion has occurred in a Cargo Bay. But when they rush to the bay to investigate, Data and Geordi are shocked to see that nothing is wrong. It seems as if there was no explosion, just a mishap with the sensors caused by Geordi's modification.
Meanwhile, several crew members begin to experience strange symptoms. Riker struggles with overwhelming exhaustion, although Dr. Crusher can find nothing wrong with him. Worf and Geordi experience sharp pains and anxiety. Even Data experiences an odd sensation when he appears to have dozed off for an hour and a half. Geordi realizes that everyone who is affected has had contact with the Cargo Bay. He orders an investigation of the premises, and soon discovers that a strange, subspace force exists in the room.
Data and Geordi inform Picard that Geordi's experiment has attracted an alien particle stream that cannot ordinarily exist in this universe. Later, they join Worf and Riker to talk about the strange sensations they have been experiencing with Troi. When they realize that all of their symptoms arose when they came in contact with a smooth, cold surface, Troi takes them to the holodeck to try to recreate the thing they fear. With some coaching, they replicate a sinister, operating-room-like environment that is lodged in all of their memories.
Data informs Picard that an intense level of tetryons exists on the starship. Even more disturbing, however, is Data's discovery that his survey indicates that he was physically absent from the ship for a period of about 90 minutes. Picard immediately asks the computer to account for all crew members, and he learns that two are missing, with no report as to where they have gone. The mystery intensifies when Beverly examines Riker and discovers that his arm has been severed and surgically reattached. Geordi and Data return to the Cargo Bay, where the tetryon levels are causing a spatial rupture. But before they can investigate further, Picard and the crew learn that one of the missing crewmembers has returned to the ship. They rush to his quarters, where the pain-crazed man dies in front of them. Realizing he may be next, Riker volunteers to wear a homing device to help the crew learn where the victims are being taken.
Later, Riker finds himself in an alien lab similar to the place recreated in the Holodeck. The other missing crewmember, Ensign Rager, is there undergoing some sort of medical experiment, obviously heavily sedated. At the same time, shadowy, alien figures remove his homing device and attempt to sedate him, but Riker only pretends to be unconscious. Meanwhile, the crew uses the homing device to determine Riker's whereabouts. The aliens create a rupture in that environment, which the Enterprise crew attempts to close. As the aliens try to counteract what the Enterprise is doing, Riker manages to grab Rager and run through the rupture the Enterprise has created. The two escape back to their ship, but with the unsettling realization that this mysterious force may strike again.