An astronaut arrives on a distant planet...a very odd one, made entirely from a thick gelid ocean. Almost immediately, odd things start to happen...
This was a tough and very dull book to wade through, as short as it was. The planet Solaris consists of a large sentient (or possibly not) ocean. For an uncounted score of years, humanity has been trying to contact it, or even discover if the ocean is alive in any meaningful sense of the word, but Solaris doesn't want to play. It ignores all attempts at contacting it and defies definition, something which apparently drives scientists to distraction and increasingly esoteric and more subtle divisions.
The lead character, Kelvin, says that Solaris denies definition, and that humanity shouldn't try. Pity the author didn't think so; fully two-thirds of the book is exposition on what Solaris might be, quoting fictional sources as though the reader knew who they were. ("Barian's theory was the predominant position until Kilkowsy's day", that sort of stuff).
As a result, we don't really care about the characters, or what they're going through. Dialogue is stilted and wooden, characters are two dimensional and we have very little understanding of them.
It picks up a star for the descriptions of the ocean beneath them, particularly vivid in descriptions of the sunsets and sunrises.