So the super-fast plot summary is: Kris Kelvin grieves for his ex-wife, Rheya, who killed herself.
And that's it.
Doesn't sound like much of a science fiction story, does it? But that's Solaris for you—it's not much of a science fiction story. Or much of a story. But it has its good points, nevertheless.
Okay, so longer version: Kris Kelvin, a psychologist, flies out to the planet Solaris, the home of a giant and possibly sentient ocean thing that humans have been trying to contact forever, but haven't managed to, because talking to a giant gelatinous ocean is harder than it looks.
When Kelvin lands on the station, he discovers that weird things are going on. His old buddy, Gibarian, has killed himself, and the other scientists, Snow and Sartorius, are acting strangely. Also, Kelvin sees a naked black woman walking through the station, who isn't supposed to be there, so there's that. (More on this lady over in the "Symbolism, Imagery, Allegory" section, if you're interested.)
Then Kelvin wakes up in his bed on the station and his ex-wife, Rheya, is there with him. This is disturbing, especially since Rheya killed herself several years before when Kelvin broke up with her. Kelvin figures this isn't the real Rheya, so he tricks her into getting into a rocket and shoots her into space. Rheya doesn't take too kindly to this, and exhibits superhuman strength while trying to get out of the rocket, but he shoots her off anyway.
Snow now explains what the deal is: Various visitors—humans or human-like creatures who seem to spring out of the scientists' memories or fantasies—have started to appear. Snow thinks it's connected to the ocean; they bombarded it with X-rays, and in return, it has sent back the visitor as a thank you… or an insult… or a test… or who knows why? The motives of a giant ocean are hard to figure.
The thing about the visitors, though, is that if you get rid of one, the ocean just provides you with another. So soon another Rheya shows up, and Kelvin doesn't shoot this one into space. No, instead he falls in love with her and decides he wants to keep her with him forever, which doesn't seem like it's going to work out very well in the long run.
Rheya, for her part, seems to love Kelvin, but slowly realizes that something is weird. For instance, if he leaves her sight, she flips out and destroys everything around her in an effort to get back to him. Pro tip: That's not quite how love is supposed to work.
Meanwhile, the scientists think they've figured out how the visitors function (it involves neutrinos) and they have various ideas for getting rid of them, including hooking Kelvin up to an EEG and beaming his brainwaves in the form of microwaves into the ocean. Yes, seriously.
What with all the talk about visitors and neutrinos and so forth, Rheya pieces things together and realizes she's an alien and tries to kill herself by drinking liquid oxygen. But she just regenerates. Which is icky to watch.
The scientists try the business with the microwaves and the brain waves, and the ocean gets frisky and makes big waves itself. Rheya sneaks away and gets Snow to shoot her with a neutrino disruptor or some such so she vanishes, and because of the thing with the microwaves and the ocean waves, or maybe for some other reason, visitors don't come back anymore and neither does she.
Kelvin is very upset that Rheya is gone, but eventually he calms down. Snow decides to stay on the station. Kelvin is waiting to get off, and in the meantime visits the ocean and has some vaguely religious thoughts.
Which is pretty much the end of the plot.
Oh, Shmoop almost forgot: The novel also includes a lot of Kelvin summarizing scientific books about Solaris, all of which say that scientists don't know anything about Solaris. In short, Solaris is not just one book in which not much happens; it is a whole multitude of books in which not much happens. The non-action is layered and multiple.