The Great Brain is Tom D. Fitzgerald, aged ten. The story is told by J.D., a sometimes confounded but always admiring younger brother. Such people as Mr. Standish, the mean schoolmaster, regret the day they came up against The Great Brain. But others, like the Jensen kids lost in Skeleton Cave, Basil, the Greek kid, or Andy, who has lost his leg and his friends, know that Tom's great brain never fails to find a way home.