For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Before we break Solaris down into a sort of beginning-middle-end framework, we'd like to be clear that Lem went to great lengths to refuse this framework with Solaris. A big part of what this book does is refuse classic plot trajectories… but we're going to squeeze it into this one anyway. It's how we do.
Act I
Some Stuff Happens
From the beginning of the book when Kelvin arrives to the end of Chapter 6, when the second Rheya shows up and Kelvin decides to treat her as if she's the original Rheya he married.
Act II
Some More Stuff Happens
From Chapter 7, when Rheya and Kelvin begin their life together on the station, to the end of Chapter 11, when the scientists complete the preparations that will allow them to get rid of the visitors.
Act III
A Little More Stuff Happens
From Chapter 12, as Kelvin waits to see the effects of the experiment, to the end of the book, when Kelvin, having lost Rheya a second time, goes out to the ocean and determines that he will wait for her to return.