A series of night-time abductions of Enterprise-D crew by solanogen-based beings from a subspace domain, who experimented on them surgically and returned them with their memories suppressed.
In The Next Generation episode Schisms, Riker cannot seem to get any rest, and Worf, La Forge and others show odd reactions to ordinary objects. With Troi's help several of them rebuild a shared half-memory on the holodeck: a cold metal table, restraints, a blade and clicking sounds in the dark.
Crusher finds that Riker's arm has been severed and reattached. The computer confirms that crew members have been missing from the ship for hours at a time. One, Lieutenant Hagler, comes back with his blood turned to a liquid polymer and dies. At the same time a subspace rupture is growing in a cargo bay, linked to changes La Forge made to the sensors.
Riker takes a stimulant to stay conscious and lets himself be taken, carrying a homing device. He brings back the missing Ensign Rager as La Forge seals the rupture. The beings' purpose is never learned.