Tired of spring-cleaning his underground home, the Mole bolts up into the sunshine and discovers the river for the first time. There he meets the Water Rat, an easygoing creature who lives for boats, picnics, and poetry, and the two become fast friends. Mole's whole world widens to the bank, its meals, its neighbors, and the slow pleasures of the water, and the book settles into the rhythm of their days together.
Through Rat, Mole comes to know the riverside society, including the wealthy, boastful Toad of Toad Hall, whose enthusiasms blaze up and burn out one after another. A passing motor-car wrecks the friends' caravan and seizes Toad with a new craze; he is left murmuring in the road, possessed, while Rat sourly explains that this is simply how Toad always is. Mole also ventures into the dangerous Wild Wood, is rescued in the snow, and is taken in by the gruff, kindly Badger, who values the security of his deep underground home above all else.
The two quietest chapters turn inward. On a winter walk Mole catches the scent of his abandoned burrow and is overcome by homesickness until Rat helps him return to it and see its small worth again. On a summer night, searching for an otter's lost child, Rat and Mole are granted a vision of a great horned Friend and Helper at the gates of dawn, an awe that holds them and then mercifully fades, leaving the riverbank touched by something sacred.
Toad's craze, meanwhile, deepens into disaster. Badger and the others try confining him at Toad Hall, but he escapes, steals a motor-car, and is sent to prison. He breaks out in a washerwoman's disguise and stumbles through a string of misadventures by canal, horse, and barge before making his way home, only to find that weasels and stoats from the Wild Wood have seized Toad Hall in his absence.
The four friends mount a final rescue. Armed and led by Badger, they slip in through a secret tunnel and rout the invaders, restoring the hall to its owner. Chastened, Toad is persuaded to give up his old swagger, hold a modest banquet, and behave at last like a reformed and grateful animal. The story closes with the riverbank world set right and the bonds of its small community affirmed.