At the start of the book, it is spring time: the weather is fine, and good-natured Mole loses patience with spring cleaning. He flees his underground home, heading up to take in the air. He ends up at the river, which he has never seen before. Here he meets Ratty (a water rat), who at this time of year spends all his days in, on and close by the river. Rat takes Mole for a ride in his rowing boat. They get along well and spend many more days boating, with Rat teaching Mole the ways of the river.