With the arrival of spring and fine weather outside, the good-natured Mole loses patience with spring cleaning, exclaiming, "Hang spring cleaning!" He leaves behind his underground home and comes up at the bank of the river, which he has never seen before. Here he meets Rat, a water vole, who takes Mole for a ride in his rowing boat. They get along well and spend many more days boating, with "Ratty" teaching Mole the ways of the river, with the two friends living together in Ratty's riverside home.
One summer day, Rat and Mole disembark near the grand Toad Hall and pay a visit to Toad. Toad is rich, jovial, friendly, and kindhearted, but sometimes arrogant and rash; he regularly becomes obsessed with current fads, only to abandon them abruptly. His current craze is his horse-drawn caravan. When a passing car scares his horse and causes the caravan to overturn into a ditch, Toad's craze for caravan travel is immediately replaced by an obsession with motorcars.
Mole goes to the Wild Wood on a snowy winter's day, hoping to meet the elusive but virtuous and wise Badger. He gets lost in the woods, succumbs to fright, and hides among the sheltering roots of a tree. Rat finds him as snow begins to fall in earnest. Attempting to find their way home, Mole barks his shin on the boot scraper on Badger's doorstep. Badger welcomes Rat and Mole to his large, cosy underground home, providing them with hot food, dry clothes, and reassuring conversation. Badger learns from his visitors that Toad has crashed seven cars, has been in hospital three times, and has spent a fortune on fines.
With the arrival of spring, the three of them put Toad under house arrest with themselves as the guards, but Toad pretends to be sick and tricks Ratty to leave so he can escape. Badger and Mole continue to live in Toad Hall in the hope that Toad may return. Toad orders lunch at The Red Lion Inn and then sees a motorcar pull into the courtyard. Taking the car, he drives it recklessly, is caught by the police, and is sent to prison for 20 years.
In prison, Toad gains the sympathy of the gaoler's daughter, who helps him to escape disguised as a washerwoman. After a long series of misadventures, he returns to the hole of the Water Rat. Rat hauls Toad inside and informs him that Toad Hall has been taken over by weasels, stoats, and ferrets from the Wild Wood, who have driven out Mole and Badger. Armed to the teeth, Badger, Rat, Mole, and Toad enter through the tunnel and pounce upon the unsuspecting Wild-Wooders who are holding a celebratory party. Having driven away the intruders, Toad holds a banquet to mark his return, during which he behaves both quietly and humbly. He makes up for his earlier excesses by seeking out and compensating those he has wronged, and the four friends live happily ever after.
In addition to the main narrative, the book contains several independent short stories featuring Rat and Mole, such as an encounter with the wild god Pan while searching for Otter's son Portly, and Ratty's meeting with a Sea Rat. These appear for the most part between the chapters chronicling Toad's adventures, and they are often omitted from abridgements and dramatisations.
