Picard is replaced by a duplicate as part of an alien experiment.
Captain Picard wakes up in an unfamiliar cell, with no idea how he got there, accompanied by three other prisoners from different worlds. Meanwhile, back on the Enterprise, a duplicate of Picard begins acting strangely, issuing odd orders and making the crew uneasy. The crew starts to suspect that something is very wrong.
The episode is a puzzle about loyalty, with Picard and his cellmates working out that they are the subjects of an experiment on how different people respond to authority and captivity. It is notable for being a story in which Picard is absent from the bridge for much of the hour, giving the crew a chance to take the lead. It is a clever, character-driven mystery.