While on a charting mission, the Federation starship Enterprise, under the command of Captain Jean-Luc Picard, discovers a patch of pure blackness in space; probes launched into the area simply disappear. As they study it further, the patch expands and soon envelopes the Enterprise, leaving them in a black void with sensors reporting nothing outside. Picard orders the ship to return on the course it entered the patch, but they find that they cannot escape; even using a stationary beacon to track their position, they find themselves brought back to the beacon.
A Romulan Warbird suddenly decloaks in front of the Enterprise, and Picard orders the crew to attack it; their photon torpedoes destroy the Warbird, but Picard suspects that it was destroyed too easily. The crew then finds a ship that appears to be the USS Yamato approaching, but does not respond to hails. Picard orders Commander Riker and Lt. Worf to beam over and search the ship. They find the ship empty, and, like the blackness, seemingly impossible with physical loops. As they are exploring the ship, the Enterprise crew spot an opening out of the darkness, but cannot lock onto Riker or Worf to transport them off the ship in time before the opening disappears. Riker and Worf are beamed back aboard as the Yamato fades away. Several similar openings appear in the blackness, each closing as soon as the Enterprise approaches them. Picard realizes that these events are tests of their responses, and orders a full-stop.
Suddenly, an entity appears in the blackness, calling itself Nagilum. It is curious about the humans and their "limited existence", and would like to test the limits of the human body, demonstrated by killing Ensign Haskell, who has violent convulsions and falls to the floor dead. Nagilum then reports that he wants to know everything about deaths and all kinds of different ways of people dying. He asserts it would only take less than half of the Enterprise's crew. Picard decides to activate the Enterprise's self-destruction sequence rather than to allow Nagilum to harm anyone else. As the crew prepares for their end, Picard is tested again by Nagilum through doppelgangers of Counselor Troi and Lt. Commander Data, both of whom question the self-destruct order. As the countdown nears zero, the void suddenly vanishes, leaving the Enterprise in normal space. Picard orders the ship to move away at high-speed and cancels the self-destruct sequence. As the Enterprise continues on its mission, Picard is met by an image of Nagilum on his ready-room computer. Nagilum offers his evaluation of humanity, criticizing the race for its faults and having no common-ground with Nagilum's kind. Picard begs to disagree, pointing out that both races are curious as shown by their recent encounter, which Nagilum agrees to before disappearing.
Data states "There is no record of any Federation vessel encountering anything remotely like this" despite the fact Captain Kirk encountered just such an area of blackness in "The Immunity Syndrome", which fits the "old Klingon legend of a gigantic black space creature which was said to devour entire vessels", that Worf recalls perfectly.
Riker misidentifies the call letters for the USS Yamato (cf. "Contagion").
When Picard is approached by the doppelgangers of Troi and Data, he is listening to one of Erik Satie's compositions, Gymnopédie No. 1 (1888).
The title of the episode is from the last stanza of a 1907 poem, The Spell of the Yukon by Robert W. Service.