Rey and Finn are separated for the entire movie
Perhaps the rarest phenomena in all cinema is the magic of on-screen-chemistry. It is an elixir impossible to generate artificially, even between gifted and accomplished actors. Filmmakers either get it, or they don’t. Luckily, the chemistry between Harrison Ford and Carrie Fisher jumped off the screen in Star Wars in 1977, and The Empire Strikes Back screenwriters Leigh Brackett and Lawrence Kasdan made the simple and obvious choice to glue those actors at the hip for their 1980 sequel.
However, lightning rarely strikes twice — no two actors in George Lucas’ entire prequel trilogy displayed even the faintest fizzle of a spark. So it goes. But, due to some miracle (and great casting), it happened again in 2015’s The Force Awakens, with Daisy Ridley’s Rey and John Boyega’s Finn making a wonderful, sparky on-screen pairing. Would they end up as a romantic coupling, a brotherly/sisterly team or as warriors-in-arms-buddies? Who could’ve said, but it was clear that the prospects for this new trilogy depended greatly on how much screen-time Daisy and John shared together.
Bafflingly then, Rian Johnson chose to keep them apart for the whole of The Last Jedi, the characters barely exchanging a word—or backward glance. The movie, and the saga, is immeasurably the poorer for it.
The First Order are tracking the fleet through hyperspace, causing the Resistance to turn on each other as suspicions run wild. Is there a First Order spy in their midst? While under attack, Leia orders the dozen cruisers and smaller ships to scatter in all directions. Meanwhile, on her ship Admiral Holdo believes the First Order are somehow tracking one of the two new Resistance recruits: either former stormtrooper Finn, or Rey via the cloaked transponders connecting Rey and Leia. Secretly — and against Leia’s wishes — she puts the transponder in Finn’s medical pod, then ejects his pod and the entire medical bay into space.
[The real explanation for how the First Order are tracking the fleet need not over-complicate matters: it should’ve had nothing to do with some sub-Star Treknobabble. Ben is simply using his Force connection to his mother to find her.]
Finn, unconscious in his medical pod, tumbles through space, eventually crashing onto a strange new world where he’s lost to everyone — except Rey.
When Rey abandons her Jedi training with Luke the first person she should find is Finn, and they’re therefore together for much of the rest of the movie.