This is a very odd episode, as most of the time it's ONLY Dr. Pholox who is in the show! It seems that the part of space they are in is toxic somehow to humans and Phlox and T'Pol man the ship by themselves as the rest of the crew are in suspended animation. However, over time, Phlox starts going batty--perhaps caused by that part of space or his overactive imagination...or both. Regardless, after a while Phlox starts seeing all sorts of things that cannot be real but totally terrify him. He's so scared that he even considers wakening some of the crew...though T'Pol manages to calm him and talk him through the worst of it. At the very end comes an amazing twist that makes this otherwise unremarkable episode well worth seeing.
In many ways, IMDb is right that the show is highly reminiscent of the episode from "The Twilight Zone" with William Shatner where he is a passenger on a plane who thinks he's seen a monster on the wing destroying the aircraft. It also is rather reminiscent of a "Star Trek: Voyager" episode where Seven of Nine is also having a lot of scary hallucinations. Regardless, it is very different and a nice change of pace for the show.