The Enterprise has been drawn to an uninhabited moon around the planet Mab-Bu VI by a weak distress signal. Data identifies it as an old Starfleet signal unused for nearly two centuries. Some quick checking confirms that it matches the Essex, a Daedalus-class ship commanded by Captain Bryce Shumar that went missing during that time period. Unsurprisingly, sensors read negative for life signs, and the hostile conditions on the surface discourage any further investigation. That is, until Troi takes a good hard sense and reports that, somehow, something is alive down there.
There is too much interference for transporters, so Riker, Data, and Troi take a shuttle down to the moon. The Screen Shake proves too much for the shuttle's engines, forcing them to crash-land. The rocky, storm-ravaged surface lends little credence to the thought of finding human survivors, but Troi is certain something is there. Something from within a strange storm front that is rapidly approaching their position...
O'Brien works his engineer magic to beam down to the surface with some pattern enhancers, which will allow all four of them to beam back up, but just before they can finish setting the enhancers up, a shock knocks the four of them to the ground. Some sort of energy enters their bodies, except for Riker, who manages to pull himself up and activate the last enhancer.
Apart from a broken arm for Riker, the away team's injuries were minimal. However, some of them are displaying odd behavior. Troi tells Picard that she felt a presence summoning her to the moon's southern polar region. Data is also uncharacteristically insistent on searching that area, and quietly sets a course even after being refused by Riker. This doesn't go unnoticed for long, which prompts Data, Troi, and O'Brien to attempt to take over the bridge by force.
This fails when Riker manages to initiate a command lockdown, so the three make a break for it. They fight their way to Ten-Forward, where they barricade themselves along with several hostages, including Worf and Keiko. They use this as leverage to get Picard to change the ship's orbit towards the moon's south pole.
Through an analysis of the transporter logs, Dr. Crusher determines that the rogue crew have been possessed by some sort of anionic energy, and that Riker may have been immune because of the pain from his fractured arm. La Forge and Ro therefore start working on a plan to drill into Ten-Forward from above with a plasma emitter that will deliver a painful but non-lethal shock that should drive the alien energy out of their hosts.
Picard negotiates for the release of several injured hostages by giving himself up in return. When he meets them in person, the possessors introduce themselves as officers from the Essex: Captain Shumar, Commander Mullen, and Lieutenant Kelly. They claim to have been trapped, disembodied, on the moon for two torturous centuries. Their request is for the Enterprise to retrieve their bodily remains and return them to Earth.
Picard doesn't believe them, not that it matters, since both sides are well past the point of establishing any sort of trust. By now, La Forge and Ro are ready to put their rescue plan into action. They line up the plasma emitter, but Data moves at the last second, barely missing the burst. He Neck Lifts Picard until the captain orders the crew to stand down.
Having reached the moon's south pole, sensors find no sign of the Essex or her crew's remains. The abductors each take a hostage—Picard, Worf, and Keiko—and arrange safe passage to a cargo bay. "Captain Shumar" reveals that they are indeed not human, but aliens who were deliberately imprisoned on the moon as a form of criminal punishment. They tried to hijack the Essex two centuries ago, and having failed that, now plan the same aboard the Enterprise. They use the cargo bay transporter to bring up hundreds more of their fellow prisoners.
However, the bridge crew is prepared for them. They trap the new arrivals on the pad with a neutrino field and Picard threatens to eject everyone into space if the three leaders do not surrender. Worf and Keiko assent, making it clear that they are ready to give their lives to save the ship. Realizing they no longer have control of the situation, the aliens release their hold on their victims, and they are all returned to their exile on the moon.
Dr. Crusher confirms that the three are healthy and free of the aliens' influence, and the episode closes with a round of apologies—gladly accepted, of course—between all those involved.
This episode contains examples of the following tropes:
- And I Must Scream:
- After it's all over, Troi describes the possession as her mind being pushed aside but still fully aware of what was going on.
- Subverted with the aliens. Worf considers the possibility that, if they really were the disembodied crew of the Essex, their long isolation could explain their violent nature, but Troi's account at the end indicates that they were of sound mind—for criminals, anyway.
- Arbitrary Skepticism: Picard does not believe that the entities are really from the crew of the Essex because no officer of Starfleet would act as they have. Apparently, Picard's never heard of the numerous captains and officers from "The Original Series" who went insane and/or turned rogue. Worf even lampshades this by noting they could have very easily gone insane after being trapped as disembodied spirits for over two centuries. While he turns out to be right in this instance, his insistence that there's no way they could possibly be Starfleet officers comes off as more naive than idealistic.
- Bottle Episode: This episode was supposed to be one, as it mostly takes place on the Enterprise. However, the producers didn't account for the many complicated special effects required, which made it one of the most expensive episodes of the season. This also happened to "The Next Phase" later this season.
- Blood Knight: The entity possessing Data is hotheaded and even tries to goad Worf into fighting him.
- Casual Danger Dialog: O'Brien, after chancing a risky beam-down to rescue the shuttle crew from an incredibly hostile environment, is chiefly concerned with getting back on time to feed Molly her lunch before the baby gets cranky.
- False Innocence Trick: Troi, O'Brien and Data are bodyjacked by noncorporeal beings. They claimed to be survivors of a Starfleet vessel that had crashed on an uncharted world about two hundred years before. They were actually convicted criminals.
- Go Mad from the Isolation: Worf theorizes this at first when the entities are posing as former Starfleet officers, to explain their very un-Starfleet-like behavior.
- Hair-Trigger Temper: The entities possessing Troi, Data and O'Brien are very on edge, and anything could set them off. The entity possessing Data is especially prone to shouting and instigating violence.
- Heroic Sacrifice: The aliens point out that if Picard goes through with his plan for dealing with them, it will kill himself, Worf, Keiko, and the officers the aliens are possessing, along with getting rid of the aliens. They are utterly unfazed.Keiko: I would die to save the life of my child.
Worf: To die defending one's ship is the hope of every Klingon.
Picard: If you each know the officers you inhabit, then you know they are equally ready to give their lives for this ship.
- Just Between You and Me: Though it probably wouldn't have mattered either way, "Shumar" wasn't doing himself any favors by telling Picard and Co. the true nature of his plan, giving them an even bigger incentive to put their lives on the line to stop him.
- Kick the Dog: The Deanna-Entity shoots down unarmed bargoers (who may or may not be civilians) in Ten-Forward even as they try to dive under tables.
- No-Sell: Phasers have no effect on the possessed officers.
- Out-Gambitted: The crew turns the hostage situation around on the entities once the remaining entities are transported into the cargo bay. A containment field is activated to keep these new entities in place, and Picard demands the release of his people—threatening to open the cargo bay to space if they don't. When "Data" points out they'd die along with them, it's made clear Picard, Worf, Keiko, and the possessed crew members would gladly sacrifice themselves to protect the rest of the ship. The entities are forced to surrender.
- Pet the Dog: During the climax, "O'Brien" takes Keiko as his personal hostage, but he allows Molly to remain behind.
- Properly Paranoid: Though he humors the entities, Picard tells Worf that he doubts the False Innocence Trick and believes Starfleet officers wouldn't behave in this manner. His idealism ends up being proven right.
- Spanner in the Works:
- A fourth noncorporeal being appears ready to go into Riker, but it flies off because he suddenly wakes up.
- A plasma shock was the initial plan to deal with the entities; it'd force them out of Troi, Data, and O'Brien, and a containment field would immediately go up. However, just as Geordi and Ro activate the device, "Data" steps away from the group at the last moment. He forces Picard to order the device turned off.
- Villain Ball: As soon as the bridge crew doesn't do as they try to convince them, "Data" and "O'Brien" turn on them and attack, and then have no real plan afterward except fleeing and trying to take control of the ship themselves. "Troi" lampshades to them that she could have convinced Picard to go to the southern pole willingly if they had just been more patient.
- Villainous Crush: "O'Brien" seems to have this for Keiko, given whom he's possessed. This doesn't stop him from taking her hostage and threatening to kill her, though.
- The Worf Effect: Worf is overcome by the being possessing Troi's body. Justified, though, as he shot "her" with a phaser, but "she" was completely immune to its effects. He, however, was not.