The Sirens of Titan opens with the scene of a materialization. A crowd is gathered around the house of Winston Niles Rumfoord, a wealthy and authoritative man who piloted a personal spacecraft into a chrono-synclastic infundibulum and now exists (along with his dog, Kazak) as a wave phenomenon, spiraling around the galaxy and temporarily materializing on planets that intercept the spiral. He appears on Earth every fifty-nine days, and his materializations are never witnessed - save for this one, that is. Rumfoord insisted that a young, unbelievably rich man named Malachi Constant be present for his next materialization, so Constant shows up.
When Rumfoord appears, he gives Constant shocking news: Constant will travel to Mars, Mercury, Earth again, and finally Titan, a satellite moon of Saturn. On Mars, he will be bred with Rumfoord's wife and produce a son, and he will end his journey on Titan, to where Rumfoord tempts him with the pictures of three astonishingly beautiful women who live there. Constant at first resolves never to go those places, but then realizing that it will probably happen regardless of what he does, he simply goes about his life until further notice.
In the following period of time, both Mrs. Rumfoord and Constant go completely broke. Mrs. Rumfoord's investments plummet and leave her with nothing, and Constant has a stroke of uncharacteristic bad luck (coupled with his wanton, extravagant spending). Constant is then recruited into the Martian Army by Martians living on Earth, where he is brainwashed into becoming a confused, mindless soldier called Unk. Mrs. Rumfoord is likewise kidnapped by Martians and taken in their flying saucer.
On Mars, Unk eventually breaks free-ish of his brainwashing, escaping the selfish exploitation of Boaz, a Martian commander who wants to live on Earth when they arrive, and deserts the army. He goes to find Chrono, his son by Mrs. Rumfoord, who rejects him. When the invasion of Mars really happens, almost everyone is killed - the reader finds out that Rumfoord is behind the invasion, and he wanted it to fail. Beatrice and Chrono land back on Earth in the jungle, and Constant finds himself stranded on Mercury with Boaz. Eventually Constant figures out how to escape, but Boaz decides to stay behind on Mercury, having grown to love the environment.
When Constant returns to Earth, he finds that he has become a religious figure in the new religion, one that Rumfoord instituted: The Church of God the Utterly Indifferent. Finding himself completely unwanted, he takes Beatrice and Chrono and takes a spaceship to Titan, where he meets Rumfoord and a small Tralfamadorian robot named Salo. It is revealed that Salo was sent millennia ago by the Tralfamadorians to deliver a message, and his spaceship broke down on Titan, so he's been stuck there ever since. He needed a replacement part, and instead of sending it to him, the Tralfamadorians manipulated the human race into developing civilization and technology in order to manufacture the part. Great monuments of the world, such as the Great Wall of China and the Kremlin, were made expressly to send inane messages to Salo; this revelation makes all of human history seem tiny and meaningless. Rumfoord disappears, and Salo takes himself apart in a sort of robot suicide.
Eventually, Constant finds Chrono's "good luck piece," a piece of metal debris that will fix the Tralfamadorian spacecraft, and reassembles Salo, who takes him back to Earth, where he quickly dies of exposure.
