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.