Commander Riker can't seem to get enough sleep at night. He's tired when he wakes up, late to his shift, exhausted all day, and when he attends Data's self-composed poetry recital, he falls asleep. (Although the joke of the recital — which hilariously includes "Ode to Spot" — is that everyone in attendance would honestly probably rather be sleeping.) But night after night, Riker lies down to bed, and then it's suddenly morning, as if the nighttime hours didn't exist.
"Schisms" is an effective, slow build of quietly escalating dread. What starts with Riker being tired slowly turns into the realization that something very ominous is afoot, and it's not just Riker. Geordi is having unexplained pain, Worf freaks out upon seeing a pair of scissors, and numerous people think they have missing hours in their lives. There is evidence that some have been subjected to surgical procedures. Eventually, a crew member turns up dead. It all culminates in a spookily played scene in the holodeck where Riker, Worf, and Geordi try to piece together foggy memories of having been ... somewhere. They remember being constrained in ... something. They slowly assemble these half-forgotten pieces into a simulated reconstruction until a strange operating table is sitting before them. It turns out they've all been kidnapped and returned by aliens from another dimension.
This episode basically takes the idea of 20th-century humans claiming to be abducted and experimented on by aliens and ports it into 24th-century Starfleet. That's a neat quasi-meta narrative trick when you think about it: The premise becomes strangely novel precisely because it doesn't seem like it should be applied to characters who are already on Star Trek.
Eventually, the crew devises a way for Riker to be abducted to the alien realm without being forced into unconsciousness, so he can stay awake and rescue a missing crew member. The aliens are strange and make clicking sounds. Sure, they look like guys wearing robes and Halloween masks, but I can't much complain. The lighting is dark and strange. The mood is bizarre and atmospheric. "Schisms" ends on a note of Unsolved Mystery. Since it seems inspired by an episode of the TV show with that name, that's appropriate.