The entire crew suffers complete memory loss after an unknown alien ship scans the Enterprise. Although unable to recall their names or duties, they quickly realize that somehow they still know how to operate the starship. In the confusion, no one notices that Kieran MacDuff, identified by the ship's computer as the Enterprise's executive officer, is someone they've never seen before. The computer also provides evidence that they are on a mission to cross into Lysian space and destroy that species's central computer, as well as any Lysian vessel that attempts to stop them. Somehow, it all seems wrong to Picard, but his new first officer insists he must follow his orders.

After being scanned by an unknown alien spaceship, the Enterprise crew discover that their memories, along with those of the ship's computer, have been partially erased. Although they retain their practical knowledge and skills, none of the crew can remember who their crewmates are, and have forgotten their own identities. Mysteriously, during the scan, an additional crewmember, in an officer's uniform, with the rank of commander, has joined the group on the bridge.

The bridge crew attempts to gain control of the situation, and Worf—wearing his baldric— assumes because he is decorated that he is the captain of the ship, and assumes command. Data, with the memory files identifying who he is unavailable, and based on where he was when the scan happened, assumes the job of bartender in Ten Forward.

After considerable time, the ship's computer memory is finally reached, and La Forge brings up the manifest of the senior staff members. Among the bridge crew is listed the mysterious new member who is identified as Commander Kieran MacDuff, the executive officer. The computer also reveals what is apparently the Enterprise's mission: According to the orders, the Enterprise is part of a fleet of vessels fighting a decades-old war with the Lysians. Their current assignment is to destroy the Lysian central command headquarters, which they are to do while maintaining communications silence.

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