The Federation starship Enterprise is sent to the inhospitable planet Nervala IV to retrieve data from a Federation research base that was abandoned about eight years earlier due to the onset of a disruption field that prevented transporter use. Commander Riker is chosen to lead the team, having been part of the rescue team that helped to evacuate the base; Riker notes that during that mission he was the last person to be beamed out to the Potemkin, where he was then serving.

Using a break in the disruption field, the away team beams down and discovers a man who appears exactly like Riker. He says that he is Riker, and has been living alone on the base for eight years ever since the Potemkin was unable to transport him back aboard, under the assumption the Potemkin crew presumed him lost. He is a Starfleet lieutenant, Cdr. Riker's rank before he was promoted as a result of this mission.

Returning to the Enterprise, Dr. Crusher determines that this person is truly a second Riker; Chief Engineer La Forge postulates that years before, when Riker was being beamed off the planet, the Potemkin had split the transporter beam to cut through the distortions, but one beam was reflected back to the base, so that Riker materialized in both places.

Cdr. Riker suggests that Lt. Riker join them on a second attempt to recover the data. At the base, their personality styles conflict, and the attempt ends in failure when Lt. Riker refuses to follow Cdr. Riker's orders. Lt. Cdr. Data postulates that the two men are resentful towards each other due to the loss of their sense of uniqueness. Lt. Worf suggests another reason: each one sees in the other something of himself he does not like.

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