The U.S.S. Enterprise returns to the planet Nervala IV to retrieve data left behind by Starfleet researchers when they were forced to evacuate eight years ago. It has taken this long for a transport window to open in the planet's distortion field, finally allowing a brief attempt for people to beam down. Riker led the evacuation operation while a lieutenant on the U.S.S. Potemkin, and returns to oversee the retrieval. However, when he beams down to the surface, he is met with an unexpected surprise — a man who appears to be his identical twin. Both Rikers claim they led the evacuation eight years ago, but while the Enterprise's Riker says he was transported back safely, Lieutenant Riker claims his transport failed, and the planet's distortion field kept him on the surface. Lieutenant Riker beams up to the Enterprise, and Beverly pronounces the two men genetically indistinguishable. Geordi then fills them in on the reason for the unusual situation - a second containment beam was initiated to get Riker transported through the distortion field, but that beam reflected back to the surface, materializing another Riker. From that moment on, two different Will Rikers — both very real — lived two separate lives.
Lieutenant Riker is ready to pick up his life where he left off — in the middle of a romance with Troi. But Troi's broken relationship with Commander Riker, who returned only to choose career over love, forced her to move on with her life. Lieutenant Riker is disappointed, but not ready to give up. Later, he agrees to help the crew retrieve the data from the computer system on Nervala IV, but when they can't access the computer core, he and Commander Riker clash over possible options during the mission. Lieutenant Riker feels resentment when the superior-ranking double gives him a direct order.
Back on the Enterprise, Lieutenant Riker romances Troi by leading her on a treasure hunt that brings them together in Ten-Forward. They talk, and Troi admits how disappointed she was when the love affair ended. Meanwhile, tension mounts between Riker and his double when Picard chooses the lieutenant's computer retrieval plan over that of the first officer. Later, with Beverly's encouragement, Troi gives in to her feelings for Lieutenant Riker, seeing his arrival as a second chance at their relationship.
Soon afterward, she talks with Commander Riker, worried that her relationship with his double will upset him. He says he accepts their romance, but warns Troi to be careful. Whatever part of his personality drove them apart since his return from Nervala IV also exists within Lieutenant Riker, and she could end up being hurt again. Troi keeps this in mind when Lieutenant Riker returns with news he's been posted on the starship Gandhi. He asks Troi to accompany him on this exciting appointment, but Troi is unsure she can give up the life she has built without him.
The two Rikers beam down to Nervala IV, entering the dangerous underground caverns to access the computer core directly. The scaffolding they attempt to cross collapses, but despite their differences, Commander Riker saves Lieutenant Riker from plunging to his death. Later, they return to the Enterprise to prepare for their rendezvous with the Gandhi. Lieutenant Riker approaches Troi for her answer, and she gently explains that she won't be coming with him — at least not yet. Commander Riker enters the lieutenant's quarters to see his double off, and gives him a memento of his past — the old trombone on which he first learned to play. Troi and the double reluctantly part, but Commander Riker promises to watch over her.