After the Crystalline Entity—which years earlier killed the colonists of Omicron Theta—attacks another colony, Picard joins forces with scientist Kila Marr to hunt it down. Marr has been searching for the entity for years, ever since it destroyed her son. But Picard's determination to find a peaceful way to stop the creature's onslaught of humanity is at odds with Marr's obsession with revenge.

Commander Riker, Data, and Doctor Crusher are visiting the Melona Four colony, when the Crystalline Entity suddenly appears and begins tearing up the planetary surface. Although the rapid evacuation is mostly successful, two of the colonists (including one in whom Riker expressed a possible romantic interest) die in the onslaught.

The Enterprise sets out in pursuit of the Entity, with the help of Kila Marr, who is a xenobiologist and expert on the creature. Marr does not trust Data, as she is aware that Data's brother Lore has assisted the Entity in the past. Captain Picard hopes to challenge this perception by having Data work closely with Doctor Marr.

Soon Marr reveals the source of her prejudice: her 16-year-old son was killed by the Entity at Omicron Theta, which was also Data's homeworld. She is initially suspicious of the android, but over time comes to understand that Data and Lore are quite different androids.

Picard points out to Marr that he does not intend to kill the Entity without first attempting to communicate with it. Marr is skeptical of this approach, but she and Data are able to work out a method for talking to the Entity.

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