So, this guy named John—who wants you to call him Jonah—decides he's going to write a book on what important Americans were doing the day the atomic bomb hit Hiroshima. Little does he know that researching the book will lead him to meet a fated group of people, to discover the religion of Bokononism, and, oh yeah, also bring about the end of the world.
But before all that, he's got a book to research.
He starts by setting up a letter correspondence with Newt Hoenikker, son of Felix Hoenikker, one of the fathers of the nuclear bomb. He then travels to Ilium to meet Dr. Asa Breed, Hoenikker's former boss. Although he makes little headway on the book in Ilium, John does talk with Dr. Breed about a substance called ice-nine. This fun material is an ice polymorph Dr. Hoenikker thought up that has the potential to destroy the world a few times over by just, you know, freezing all the water on Earth.
After his trip to Ilium, John all but forgets about the book until he's sent on assignment to San Lorenzo. He's there to interview the charitable doctor Julian Castle, but by either coincidence or fate, he also meets a host of other characters including Mr. and Mrs. Crosby, the Mintons, and all three of the Hoenikker children. He is also introduced to Bokononism for the first time.
At San Lorenzo, John learns that the island's current dictator, "Papa" Monzano, is dying of cancer and Frank Hoenikker, Felix's eldest child, is his successor. But Frank offers the job of President of San Lorenzo to John instead. Why? Because Frank doesn't want the job, so it might as well go to someone. And, hey, why not John? He accepts the job kind of to help the poor people of San Lorenzo but mostly so he can marry the beautiful Mona Monzano.
Once the line of succession is secure, "Papa" Lorenzo kills himself by consuming none other than ice-nine. John forces the Hoenikker children to confess that their father created the substance, and they've given the stuff away to get what they wanted from life. John and the Hoenikkers attempt to clean up the ice-nine mess to cover up its existence, but a freak accident during a flight demonstration destroys the castle, sending "Papa" Monzano's corpse into the sea.
What happens next is an instant ice age spreading the entire globe.
Initially, John and Mona stay in Monzano's oubliette to escape the ice-nine apocalypse, but after Mona commits suicide, John shacks up with Newt, Frank, and the Crosbys. While deciding what exactly to do with his life now that the world has ended, John meets up with the legendary Bokonon, who suggests someone should write a book on the history of human stupidity.
Surprise! That's the book you've just read.