The Enterprise is assigned the mission of visiting planet Nervala IV to retrieve some important scientific data. The information had been lost when a previous research team was forced to abandon the post. A nearly impenetrable distortion field makes use of the transporter impossible most of the time and problematic at best.
Commander Riker heads the away team. When the previous research team was rescued, Riker was serving as a lieutenant on the USS Potemkin, the ship which carried out the rescue; in fact Riker had been the last person to leave the science station.
Deep in the base, Riker and the away team confront Riker's doubleâa man who believes he had been left behind eight years ago instead of being beamed up to the Potemkin. He has been alone ever since.
Back in sickbay, Dr. Crusher verifies the obvious: The man is an exact double of Riker down to the cellular and genetic level; even their brain patterns (which are formed by experience and can't be cloned) are a match.
After examining the Potemkinâs transport logs, Geordi comes up with an explanationâyears earlier, as the transporter officer of the Potemkin struggled to beam up Riker through the distortion field, he tried the novel approach of splitting the transport beam. Part of the transport beam was reflected by the distortion field back to the planet, materializing an exact living replica of Riker. Unaware of what had happened, and assuming they had successfully recovered Riker, the crew of the Potemkin went on to their next assignment leaving the duplicate trapped behind.
'Lieutenant' Riker volunteers to accompany a second away mission attempting to recover the important research data. The attempt fails and the two Rikers clash as their philosophical styles differ greatly. Finally Commander Riker orders Lt. Riker to cooperate; having a double who is a superior officer makes Lt. Riker very resentful and angry.
Returning to the ship, Commander Riker struggles to rationalize his instinctive resentment of the lieutenant; Data supposes that each man is unsettled and feels somewhat violated that the other has intruded upon his âuniquenessâ as a person. In addition, it appears that some of the Commander's dislike comes from dealing with another earlier version of his personality which he sees as less mature and settled; he doesnât like being reminded that he was once much like Lt. Riker, a facet of himself which he had put aside years earlier.
Both men propose ideas to Captain Picard for a third attempt at the retrieval of the missing data; Picard causes further discord between them when he chooses to pursue Lt. Rikerâs plan.
Meanwhile, Lt. Riker has set about the business of moving on with his life and catching up on the last several years. Stranded on the station (unlike Commander Riker), he didn't choose to sacrifice his relationship with Counselor Troi to concentrate on his career in Starfleet, and he attempts to pick up their love affair right where it left off when he was stranded on Nervala. Troi realizes that she is still regretful that things didnât work out with Commander Riker in the past, and with Crusherâs encouragement promotes Lt. Rikerâs advances, seeing this as a second chance at a relationship that failed once before. However, as Lt. Riker continues to chafe at the frustration of having to deal with a senior officer who is a double of himself, he proposes taking Troi away with him to a new posting on the starship Gandhi. Deanna is ambivalent at the prospect of leaving her life aboard the Enterprise, and tells him she will have to think about it.
Another attempt to gain the information goes badly. A walkway breaks, endangering the life of Lt. Riker. Despite him telling the Commander to 'let go', Riker saves his double.
Back on the ship, both Rikers are more accepting of each other as Lt. Riker prepares to leave for his new posting. The commander gives his double a cherished trombone. Lt. Riker decides to take the name of Thomas to be unique, a name he likesâunlike his double. Deanna tells Thomas she has decided not to transfer with him.