The Sirens of Titan is a complex, layered, humorous, and yet darkly satirical novel that makes the reader laugh out loud while leaving a bitter taste in his mouth (not unlike most of Vonnegut's work). On its most basic, material layer, it is a science fiction adventure story about a young rich human who, having fallen on hard times, is brainwashed into joining a Martian army and invading Earth, has a child with his prophet's wife, deserts and crash-lands on Mercury, returns to Earth and is banished to Titan, where he meets a small alien robot who tells him the secrets of life and the universe.
A deeper reading, however, reveals underlying philosophical arguments that the average reader might find unpleasant and even disturbing. The most obvious deep reading of this novel's plot is an argument for the senseless, arbitrary randomness of humankind. All of the novel's events occur suddenly, for little to no reason, in a manner that upends the characters' lives on the basis of pure randomness: for example, Constant happens to be recruited by the Martian army when he's feeling low, and suddenly he's a brainless soldier on Mars with no critical thinking skills, being bred to produce children and used as a pawn in the war against Earth. Such arbitrary leaps pervade the novel, making it both hilarious and entirely disconcerting. This message is especially obvious when Constant arrives on Titan and speaks with Salo the Tralfamadorian robot, who informs him that the entirety of human history was enacted and controlled by the Tralfamadorians in order to produce a particular piece of scrap metal to fix Salo's spacecraft. Salo had been stranded on Titan after being sent with a message for humanity: a single dot, meaning "Greetings."
On another level, The Sirens of Titan can be read as a satirical polemic against organized religion. Christianity is caricatured in Constant's father's use of the letters of an English translation of Genesis as random indicators of which companies to invest in, resulting in an avalanche of wealth, parodying the idea of the "prosperity gospel," the belief that trust in God will result in material comfort. Most Christians reject this idea with contempt, but that doesn't stop Vonnegut from using it as a straw man on which to take out his feelings. This mockery is even more obvious is Rumfoord's establishment of a new religion, "The Church of God the Utterly Indifferent." The satire is self-explanatory: God seems to not care what happens in the world, so it's better to just come out and say it unabashedly. Constant is also portrayed as a sort of Christ-figure, being the principal figure in Rumfoord's religion, and being forced to board a spaceship (in a manner resembling crucifixion) and "ascend" into the heavens, flying to the Jovian moon Titan. This elaborate parody, coupled with Constant's following revelation that human life is essentially pointless, finalizes Vonnegut's attack on organized religion.
