Riker meets a duplicate of himself left behind eight years ago.
Riker returns to Nervala IV, where he served eight years earlier, to retrieve data, and he discovers that a transporter accident created a duplicate of himself. The second Riker has been stranded on the planet since the accident, and he has lived a very different life. The two men must confront each other.
The story is a fascinating thought experiment about identity, and it asks what makes a person who he is. The duplicate, who calls himself Thomas Riker, has stayed true to the man that Riker used to be. The episode is a clever character study, and the second Riker returns in a later series. It is a popular and thought-provoking entry.