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 envelops the Enterprise, leaving them in a black void with sensors reporting complete nothingness outside. Picard orders the ship on a return course, but they find that they cannot escape; they leave a stationary beacon behind them, only to have it reappear ahead of them again.
A Romulan Warbird suddenly decloaks in front of the ship and attacks, and Picard orders the crew to return fire; they destroy the Warbird, but Picard is suspicious of how easily. The crew then detect what appears to be their sister ship, the USS Yamato, approaching, but it does not respond to hails. Commander Riker and Lt. Worf beam over to search the ship, where they find it empty, with inconsistencies in its construction, including more seemingly impossible physical loops. The Enterprise then detects an exit from the darkness, but cannot lock onto the away team to retrieve them before the opening disappears. The Yamato begins to fade away, but the Enterprise is able to beam Riker and Worf back just in time. More openings appear in the blackness, each closing as soon as the Enterprise approaches them. Picard realizes that they are being manipulated, and orders a full-stop.
Suddenly, an entity appears, calling itself Nagilum. It is curious about the humans and their "limited existence", and would like to test the limits of the human body. It causes Ensign Haskell to experience violent convulsions and fall to the floor dead. Nagilum then states that he wants to know everything about death, asserting that it would take less than half of the Enterprise's crew to complete his experiments. Picard decides to activate the ship's self-destruct sequence rather than submit to Nagilum's further experiments. 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 when he is finally satisfied that they are truly free, 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's faults and claiming they have nothing in common with his kind. Picard disagrees, pointing out that their recent encounter shows that both races are curious, which Nagilum concedes 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.