One morning, while pondering the stress of his latest assignment at his uninspiring job, the narrator of Kangaroo Notebook feels an itching on his leg that seems to indicate an unusual hair loss. The next morning he wakes to discover that he is sprouting small radishes on his shins. After battling to be seen in his local medical clinic, he enters a hospital, where a physician prescribes hot-spring therapy in Hell Valley.
Hooked to a penile catheter and an IV bottle, the narrator begins a harrowing journey on his hospital bed through the underworld that seems to lie beneath the city streets. Here, he seeks health not so much as he seeks simple explanations for what is happening to him and the strange people he meets: abusive ferreymen, waiflike child demons, vampire nurses, a chiropractor who runs a karate school and works a sideline as a euthanist.
The prelude describes a tidal wave approaching Japan. The story is a first-person narrative by Professor Katsumi, inventor of a self-programming computer which can predict the future. Katsumi and his assistant, Tanamogi, plan to predict the future of a private, individual destiny. They choose a subject from the street and follow him. The next day's paper announces his murder.
To solve the case and forestall suspicion, Katsumi downloads the contents of the man's brain, reconstructs his existence, and questions him/it. The victim did not see his murderer, but he tells the team his mistress had sold her aborted foetus for 7000 yen. Then the mistress is murdered. Katsumi's wife has a forced abortion and receives 7000 yen. Katsumi suspects an organization. His assistant Tanamogi volunteers the name of an organization experimenting with extra-utero development of foetuses, and arranges for Katsumi to visit their lab. Gradually Katsumi learns of a vast conspiracy to create an underwater nation, complete with genetically altered water-oxygenating humans and animals, bred in anticipation of the predicted destruction of Japan by a tidal wave.
