Psychologist Kris Kelvin is being sent on an interstellar journey to evaluate whether a decades-old space station should continue to study the oceanic planet Solaris. He spends his last day on Earth with his elderly father and retired pilot Berton. Years earlier Berton had been part of an exploratory team at Solaris but was recalled when he described seeing a four-meter-tall child on the surface of the water. This was dismissed as a hallucination by a panel of scientists, but now that the remaining crew members are making similarly strange reports, Kris’s skills are needed.
Upon his arrival at Solaris Station, a scientific research station, none of the three remaining scientists bother to greet Kelvin, and he finds the space station in dangerous disarray. He soon learns that his friend among the scientists, Dr. Gibarian, has killed himself. The two surviving crewmen—Snaut and Sartorius—are uncooperative and evasive. Kelvin catches fleeting glimpses of others aboard the station who were not part of the original crew. He also finds that Gibarian left him a rambling, cryptic farewell video message, warning him about the station.
After a fitful sleep, Kelvin is shocked to find Hari, his late wife, in his sleeping quarters. She is unaware of how she got there. Terrified by her presence, Kelvin launches the replica of his wife into outer space. Snaut explains that the “visitors” began appearing after the scientists conducted radiation experiments using X-rays in a desperate attempt to understand the planet’s nature.
That evening, Hari reappears in his quarters. This time Kelvin calmly accepts her and they fall asleep together in an embrace. Hari panics when Kelvin briefly leaves her alone in the room, and injures herself. But before Kelvin can give first aid, her injuries spontaneously heal before his eyes. Sartorius and Snaut explain to Kelvin that Solaris created Hari from his memories of her. The Hari present among them, though not human, thinks and feels as though she were. Sartorius theorizes that the visitors are composed of “neutrino systems” but that it might still be possible to destroy them through use of a device known as “the annihilator”. Later, Snaut proposes beaming Kelvin’s brainwave patterns at Solaris in hopes that it will understand them and stop the disturbing apparitions.
In time, Hari becomes independent and is able to exist away from Kelvin’s presence. She learns from Sartorius that the original Hari had committed suicide ten years earlier. Sartorius, Snaut, Kelvin and Hari gather together for a birthday party, which evolves into a philosophical argument, during which Sartorius reminds Hari that she is not real. Distressed, Hari kills herself again by drinking liquid oxygen, only to painfully resurrect after a few minutes. On the surface of Solaris, the ocean begins to swirl faster into a funnel.
Kelvin becomes ill and goes to sleep. He dreams of his mother as a young woman, washing away dirt or scabs from his arm. When he awakens, Hari is gone; Snaut reads her farewell note, in which she describes how she petitioned the two scientists to destroy her. Snaut then tells Kelvin that since they broadcast Kelvin’s brainwaves into Solaris, the visitors had stopped appearing and islands began forming on the planet surface. Kelvin debates whether or not to return to Earth or to remain with Solaris.
Kelvin meets with his father at their dacha. It is later revealed to be on an island in the Solaris ocean.