the story: Riker discovers his transporter duplicate.
similar to: "Defiant" (Deep Space Nine)
my thoughts: As opposed to episodes where a character is split apart (the original series' "The Enemy Within" or Voyager's "Faces") or simply melded together (Voyager's "Tuvix"), "Second Chances" has the distinction of introducing a permanent alternate version (not counting the Mirror Universe, naturally) of a main character.
Transporters accidents are a classic franchise trope, and on that score "Chances" may be the prototypical example, in some ways the nightmare scenario even worse than somehow dying from one (although that happened in The Motion Picture, too). But most significantly, not only does Thomas Riker come into existence, he actually returns later, too (Deep Space Nine's "Defiant"). That means everything this episode concludes had an additional layer of resonance.
"Chances" might be considered a what-may-have-been story (Voyager is chalk-full of those!), from the unique perspective of it actually playing out. Thomas Riker (the transporter duplicate) was created before Will Riker became the man we're familiar with (in some ways, "Tapestry" without the reset), which means, among other things, the famous Riker/Troi romance that was one of the worst teases of the series gets to be revisited, and in some ways addressed directly for the first time.
Although the episode somewhat glosses over how Thomas was able to survive on his own for as long as he did (unlike those pesky Soong androids, no reassembly required!), it's a nifty reminder that nearly everyone in this series has a similar origin story, where they were literally scooped up from the ruins of a shattered life, and had to somehow make good and start anew. You can actually enjoy "Chances" for that peek into all the stories we mostly only really heard about over the years.
Because I don't think "Second Chances" is a classic, I'm going to classify, besides its distinction as an essential character piece for Riker, as a series recommendation over a franchise one, although this is kind of splitting hairs. Bottom line, it's a 3-star affair.
Jonathan Frakes (Thomas Riker)