The Enterprise arrives at a barren world that's the site of a former Starfleet installation that was abandoned eight years ago because the planet's distortion field rendered access to the surface impossible by both shuttle and transporter. With brief windows of entry over the next few days, the Enterprise hopes to retrieve data from the station's computers before the opportunity for access expires. Crucial detail: Riker was part of the original evacuation mission eight years ago when Starfleet abandoned the post and was just barely beamed out before the window in the field closed completely.
So the crew is stunned to find a doppelganger of Riker on the station who claims to have been trapped there for eight years. It turns out the planet's distortion field caused Riker's transporter beam to be copied and reflected back to the surface, essentially cloning him (memories and all) into two separate people who are equally and legitimately William T. Riker. Neither knew that the other existed.
It's another high concept, to be sure, but it's a brilliant one the story takes seriously and uses to explore the past and present of Riker's character better than nearly all TNG episodes that have ever centered on him. Here's an episode that stops to consider that when you come face to face with someone who claims to be you and the proof shows he's not lying, a piece of you feels like your identity has been stolen. While this version of Riker seems a little too well-adjusted (and, indeed, not as different from our Riker in general as he probably should be) given that he has spent the past eight years in total isolation, the story uses its premise to posit a series of what-ifs (hence the title) — taking a look at the choices Riker made eight years ago, and asking whether he would make them again.
Most crucially is the question of Riker and Troi. If you look at the six years of TNG's run to this point, you realize that a lot was implied and hinted at about Riker and Troi and their relationship before they were both assigned to the Enterprise — indeed, the most concrete dialogue might actually have been in "Encounter at Farpoint," regrettably. But "Second Chances" finally takes a look at these two people and acknowledges that they once had an important and real relationship, and uses that to tell the rare romantic story on TNG that actually, truly works — precisely because it has a real history and real stakes and is believable, rather than being concocted in a few hours or days and banished to the realm of the immediately irrelevant, like most one-off romances.
This Riker spent many of those first few trapped months holding on to the hope that he would be rescued and ultimately reunited with Deanna — and when he learns that after the very mission that left him stranded his duplicate went on to make his career the priority, he finds it almost impossible to believe. He is certain he would never make that choice himself. But the situation allows them to perhaps pick up where they left off, and Troi herself allows for the possibility. "Second Chances" is essentially an exploration of the age-old adage of The Road Not Traveled, and a surprisingly effective one with good dialogue and character moments. One man went on to choose his career over his relationship, while the other one spent eight years trapped in a hole. Now seeing the big picture, the guy who was trapped in the hole has some resentments.
This culminates in a very good scene where Riker plays a poker hand against himself and the two of them clear the air. It's a simple and grounded scene dealing with an extraordinary situation. But naturally, you can't go home again; there are too many complications for the romance here to continue, and ultimately we can't have two Rikers living aboard the Enterprise. But I respect the decision by the writers not to make the obvious move of killing Thomas Riker and instead having him transferred to another ship to continue his life. (And this would, of course, have its own intriguingly bizarro consequences in DS9's third-season episode, "Defiant.") "Second Chances" is an episode that could've been a cynical or shallow exploration of its premise, but the execution is anything but.