An away team boards the USS Defiant to find the crew all dead.
This is a strong character episode that has a few story issues but is well made.
The plot has a decent ticking clock premise and contains a few interesting concepts, slightly too many, but it is told in an entertaining way due to the focus on characters.
Spock and Dr McCoy have most of the limelight, as Captain Kirk is absent for a large part of the episode and their dynamic is quite compelling. Bones is slightly infuriating at times, but that makes it all the more satisfying when Spock puts him in his place with some calmly delivered reminders of who's in charge.
U'Hura, Chekov and Scotty have some decent moments, particularly Nichelle Nichols getting a rare opportunity of do more than just open hailing frequencies. James Doohan has some funny moments and so does Walter Koenig (albeit some unintentional).
Visually it works wonderfully well, as there is a lot of storytelling done with the camera. I love the scenes on the Defiant, and the odd camera angles indicating the perspective of characters suffering space madness. The effects used for the web and the interspace phasing are also good.