Overview: The Enterprise becomes enveloped by a void in space where the crew are tested by a powerful alien presence.
On stardate 42193.6, the USS Enterprise is on a charting mission in the Morgana Sector, when Ensign Crusher soon picks up a strange cloud of pure blackness floating in space. He puts the anomaly on the main viewer noting that it visually blocks out the stars behind it. Captain Picard orders a probe to be launched, but as soon as it enters, it disappears. This alarms Worf who describes an old Klingon legend about a space creature that "devours entire vessels". Picard orders a second probe, but it too disappears upon entering.
Suddenly, the blackness expands and surrounds the Enterprise, but causes no harm. In the void, no stars are visible, but Picard decides to hold position and collect whatever information they can. Unfortunately, Data indicates that the sensors detect nothing and that the void has no mass or dimension, and logically shouldn't exist. Picard orders an exit course, but as the ship travels, there is no end in sight and the space appears to be caught in a loop. Data drops a stationary beacon to use as a positional positional reference. At warp, the signal rapidly fades behind the ship, but is soon picked up again dead ahead – proof the ship is running in circles.
Suddenly, sensors detect a Romulan Warbird decloaking and Picard orders Red Alert. The Romulan ship appears and fires a volley of torpedoes that wrack the Enterprise and Picard orders return fire. Curiously, the weapons quickly vaporize the enemy and made it seem too easy. Next, sensors detect an incoming Federation vessel – the USS Yamato (NCC-1305-E) – the Enterprise's sister ship. Strangely, the craft ignores hails. Commader Riker offers to beam over to the Yamato and investigate, taking Worf with him. There, they find no one aboard, and amongst other strange things, the ship has not one, but three bridges, that are seemingly linked together in looped dimension.
On the Enterprise, Data spots an opening in the void showing normal space beyond. It's an opportunity to leave, but transporter Chief Miles O'Brien has trouble bringing Riker and Worf back. To make matters worse, the Yamato begins to fade away. O'Brien manages to return Riker and Worf just as the Yamato vanishes and Riker angrily returns to the bridge to vent his frustrations at being toyed with. Soon another opening forms, but as the Enterprise races toward it, it closes up. Again another opening appears and another, and each time they close before the ship can reach them. Despite another opening forming, Picard orders a full stop.
Counselor Troi, who sensed nothing beforehand, now detects a curious intelligence lurking nearby. A moment later, a shocking visage appears on the viewer. With a booming voice, it identifies itself as Nagilum. Picard tries to be friendly toward the entity, but it ignores him and begins asking direct questions about human sexual reproduction and making the observation that Data, although appearing human, is simply a machine.
Still annoyed by having been toyed with, Picard tells Nagilum that, his previous actions were not welcome. Nagilum ignores Picard's and instead displays curiosity as to why humans have a "limited existence" which ends in death. Without warning, the ship's conn officer, Ensign Haskell, begins to convulse violently. With a scream he collapses in a fetal-position on the floor. Dr. Pulaski rushes to help him but he is dead. Nagilum watches with fascination as Picard reacts with anger, shouting that he cannot allow him to do that.
Nagilum states that he wishes to understand more about death and needs to conduct experiments that shouldn't take more than a third of the crew – or maybe half. The entity vanishes leaving Picard with no choice against it. He decides that whatever happens they shouldn't stand by idly and allow it. With only one decision, Picard orders auto destruct of the Enterprise hoping this will stop Nagilum from harming anyone else. Riker agrees with his decision and decides on a 20-minute countdown.
With time ticking down, Picard goes to his quarters and listens to Eric Satie's "Gymnopédie". Soon Counselor Troi solemnly enters, followed by Data. Troi warns that blowing the ship up won't stop Nagilum, while Data poses questions about what death really is and if there is an existence beyond it. Picard admits that the universe is vastly complex and he is unable to say for sure if there is life after death, but he believes there is something more to experience out there. Given that, Data responds that they should not destroy themselves since Picard is uncertain of their destiny. Troi likewise states that the decision to destroy everyone is not his alone to make and demands they should be given a chance at not being picked for Nagilum's experiments.
Knowing his people better than this, Picard become suspicious by this talk and asks the computer for the location of Data and Troi. When the computer says that they are on the bridge, Picard realizes he has been dealing with dopplegangers and calls Nagilum's bluff. The dopplegangers vanish and Riker calls to announce the void has vanished. With seconds remaining on the countdown, Picard orders a random course change to anywhere. Everything seems normal enough, and to Riker's relief, Picard cancels auto-destruct at the last seconds.
Picard returns to his ready room hoping that Nagilum got what he needed. Nagilum's visage suddenly appears on the desk screen where he offers an "evaluation" of humanity. Picard listens as Nagilum says mankind finds no tranquility in anything, struggles against the inevitable, and thrives on conflict. He also states that humans are selfish, aggressive, rash, quick to judge and slow to change. He concludes that, as a species, humans have no common ground with his kind. Picard doesn't disagree with any one point, but says that curiosity is a trait that both species seem to share. With a human-like chuckle, Nagilum agrees. Picard says that they may meet again and hopes it will be among the stars. Nagilum fades off screen. Picard returns to the bridge, and Riker tells Wesley to "steer clear of any holes" from now on.