The title refers to a distant planet, which is covered with an ocean of plasma. More than a century before the events of the novel, Earth scientists discovered, that the entire ocean is one, living, intelligent organism; however, every attempt to establish communication with it was futile.
At the beginning of the novel, psychologist Kris Kelvin (the narrator of the story) arrives to Station Solaris, a scientific research station hovering near the surface of the planet. He discovers that the leader of the research team, Gibarian killed himself, and the other two members, Snow (Snaut in Polish) and Sartorius are acting strangely. He soon realizes why, when a Doppelganger of his dead wife, Rheya (Harey in Polish) appears in his room. Turns out, that the ocean sent such replicas (called "visitors") to every member of the team, for unknown reasons. Those "visitors" presumably represent their greatest failures; Rheya killed herself when Kelvin left her, and he still feels guilty about that. Kelvin first lures the Rheya visitor in a shuttle, and launches it into outer space, but she soon reappears, with no memory of the incident. A conflict appears between the members of the team; Kelvin wants to live with Rheya, while Snow and Sartorius want to get rid of the visitors for good.
The novel was adapted to film two times, by Andrei Tarkovsky in 1972, and by Steven Soderbergh in 2002. The first film is considered a classic. Lem disliked both versions, claiming that they focus on the humans too much, and miss the actual theme of his novel.
- Alien Geometries: The symmetrids, and asymmetrids, are giant formations consisting of a bizarre keratin-like substance. They appear from the black ocean, exist for a period of time, and then collapse back into the sea. Symmetrids are perfectly symmetrical down to the molecule, and asymmetrids are chaotic, unstable and only exist for a fraction of the time of the former. They're described as performing some sort of computer-like calculation process within their own machine-like bio-structure, but towards no understandable or observable purpose.
- Cosmic Horror: One possible interpretation of the planet, though it's important to note that we can't know if it is malicious or simply so alien in its workings that it becomes terrifying. It's sentient, but its thoughts and motives are beyond comprehension, as are its physics: somehow, it can affect the workings of the universe on an astronomical scale, but no one knows how. At the time of the novel, humanity has been studying Solaris for a century with barely any progress, and many attempts to communicate directly with Solaris have... unpleasant results.
- Driven to Suicide: The original Rheya and Gibarian. The second replica of Rheya also tries to kill herself, by drinking liquid oxygen, when she learns what she is, but she survives due to her healing factor. Eventually, Sartorius and Snow destroy her with a device that disrupts her sub-atomic structure at her request.
- Genius Loci: The planet.
- God Is Inept: At the end, Kelvin theorizes about a god "whose imperfection represents his essential characteristic: a god limited in his omniscience and power, fallible, incapable of foreseeing the consequences of his acts, and creating things that lead to horror." Snow suggests that the ocean might be the first phase of such a god.
- Healing Factor: The visitors have it; it's impossible to kill them, their wounds heal in moments.
- Humanoid Abomination: Of a sort, in the form of a giant baby.
- Jerkass: Sartorius.
- Last-Name Basis: The members of the research team. When Snow once calls Kelvin on his first name, he feels grateful for it.
- Living Memory: Rheya's replica.
- Minimalist Cast: The only characters in the novel are Kelvin, Snow, Sartorius and Rheya... and she isn't a real person. Though she appears twice; does that count as two characters? Kelvin also briefly sees Gibarian's visitor, a large black woman.
- My Greatest Failure: Kelvin regards his failure to stop Rheya's suicide as this; that's why she appears for him. Probably.
- No Name Given: The first names of Snow and Sartorius, the last name of Rheya.
- Offscreen Teleportation: How the visitors appear.
- Outgrown Such Silly Superstitions: When Kris asks Snow if he believes in God, he responds: "Who still believes nowadays..."
- Pinch Me: When Kelvin first sees Rheya, he thinks he's dreaming. When he wants it to end, he stabs his leg with a spindle. But it's not a dream.
- Riddle for the Ages: Why did the ocean sent the visitors? And why did it stop? Was it a test? Was it torture? Was it a misguided attempt of a good deed? The point of the novel is that we can never know.
- It's strongly hinted that it has something to do with one of the strange formations seen on the surface (the "symmetriad")--that the ocean was trying to understand humans in terms of one, and the "ghosts" were the ocean trying to generate the individual humans' "missing" counterparts.
- Also, what visitors did Snow and Sartorius get? They're probably manifestations of sexual fantasies, but it's never made clear.
- Starfish Aliens: An entire ocean of sentient plasma.
- Despite looking human, the visitors might also qualify, since their subatomic structure is completely alien.
- Super Strength: The visitors have it, as Kelvin soon learns.
- Uncanny Valley: The visitors definitely count, such as the protagonist's girlfriend's dress having buttons but lacking any seams or even a way for it to be put on or removed. There are also biological formations out in the ocean, made out of a calcified substance, that mimic the appearance any number of things from human-looking buildings and trees to people and dogs. They don't last long and are eventually reabsorbed into the "water."
- Not to mention the description of the giant baby which does not act in any way baby-like, but instead systematically tests out its body, to the horror of the witness.