The Sirens of Titan is the story of wealthy and fortunate 22nd-century industrialist Malachi Constant. The science-fiction novel deals with a Martian invasion of Earth, Constant's quest to find the source of his seemingly inexplicable luck, and his ruminations on ideas like human agency, the all-knowing nature of God, and the purpose of the existence of the human species. Constant survives the conquest of Earth and becomes an intergalactic wanderer drawn into a much larger mystery.
Malachi Constant is the richest man in a future America. He possesses extraordinary luck that he attributes to divine favor which he has used to build upon his father's fortune. He becomes the centerpoint of a journey that takes him from Earth to Mars in preparation for an interplanetary war, to Mercury with another Martian survivor of that war, back to Earth to be pilloried as a sign of Man's displeasure with his arrogance, and finally to Titan where he again meets the man ostensibly responsible for the turn of events that have befallen him, Winston Niles Rumfoord.
Rumfoord comes from a wealthy New England background. His private fortune was large enough to fund the construction of a personal spacecraft, and he became a space explorer. Traveling between Earth and Mars, his ship—carrying Rumfoord and his dog, Kazak—entered a phenomenon known as a chrono-synclastic infundibulum , which is defined in the novel as "those places ... where all the different kinds of truths fit together." When they enter the infundibulum, Rumfoord and Kazak become "wave phenomena", somewhat akin to the probability waves encountered in quantum mechanics. They exist along a spiral stretching from the Sun to the star Betelgeuse. When a planet, such as the Earth, intersects their spiral, Rumfoord and Kazak materialize, temporarily, on that planet.
When he entered the chrono-synclastic infundibulum, Rumfoord became aware of the past and future. Throughout the novel, he predicts future events; unless he is deliberately lying, the predictions always come true. It is in this state that Rumfoord established the "Church of God the Utterly Indifferent" on Earth to unite the planet after a Martian invasion. It is also in this state that Rumfoord, materializing on different planets, instigated the Martian invasion, which was designed to fail spectacularly. On Titan, the only place where he can exist permanently as a solid human being, Rumfoord befriends a traveller from Tralfamadore (a world that also figures in Vonnegut's Slaughterhouse-Five, among several others) who needs a small metal component to repair his damaged spaceship.
Salo, the Tralfamadorian explorer, is a robot built millennia earlier to carry a message to a distant galaxy. His spacecraft is powered by the Universal Will to Become, or UWTB, the "prime mover" which makes matter and organization wish to appear out of nothingness. (UWTB, Vonnegut informs the reader, was responsible for the Universe in the first place, and is the greatest imaginable power source). A small component on Salo's spacecraft breaks and strands him here in the Sol System for over 200 millennia. He requests help from Tralfamadore, and his fellow Tralfamadorians respond by manipulating human history so that primitive humans evolve and create a civilization in order to produce the replacement part. Rumfoord's encounter with the chrono-synclastic infundibulum, the following war with Mars, and Constant's exile to Titan were all manipulated via the Tralfamadorians' control of the UWTB. Stonehenge, the Great Wall of China and the Kremlin are all messages in the Tralfamadorian geometrical language, informing Salo of their progress.
As it turns out, the replacement part is a small metal strip, brought to Salo by Constant and his son Chrono (born of Rumfoord's ex-wife). A sunspot disrupts Rumfoord's spiral, sending him and Kazak separately into the vastness of space. An argument between Rumfoord and Salo moments before concerning the contents of Salo's message, left unresolved because of Rumfoord's disappearance, leads the distraught Salo to disassemble himself, thereby stranding the humans on Titan. It is revealed that the message was a single point, meaning 'Greetings' in Tralfamadorian. Chrono chooses to live among the Titanian birds; after thirty-two years, his mother dies, and Constant manages to reassemble Salo. Then, using the part delivered so many years previously by Chrono, Constant repairs the Tralfamadorian saucer.
Salo returns Constant to Earth where he dies of exposure in wintertime Indianapolis while awaiting an overdue city bus. But as he passes away he experiences a pleasant hallucination secretly implanted in his mind by a compassionate Salo.