The novel, narrated by the Englishman J., tells of a boat trip J. takes up with Thames River with his friends George and William Samuel Harris. His prose is rambling, and often digresses into anecdotes or long observational passages.
One night, the three men smoke together in J.'s London apartment, discussing their anxiety over their sicknesses. The reader can discern that they are actually hypochondriacs. After researching diseases at the British Museum, J. has recently concluded that he suffers from every disease known to man except for housemaid’s knee. The men decide that a vacation will be good for their health, and after some deliberating, they decide to spend a week rowing up the Thames with their dog, Montmorency.
The men make arrangements for the trip. They decide to bring a cover for the boat so they can sleep in it, rather than bringing a tent or staying in inns. They compile a long list of items to bring, but quickly realize that they should only pack the essentials. Although they are friends, J. seems to dislike Harris, and compares him at length to J.’s incompetent Uncle Podger. They end up bringing a hamper of food, clothing, a cover for the boat, and a methylated spirit stove for cooking. Packing takes a long time because the men keep forgetting items they need, and prove somewhat ill-fit for the the task.
The men oversleep on the morning they are supposed to leave, and have trouble determining which train to board for Kingston, from which they intend to embark. They eventually make it, though, and begin the journey. J. describes some local landmarks, including Hampton Court and some pubs that Queen Elizabeth dined in. Harris tells a story about getting lost in the hedge maze at Hampton Court.
The men pass through their first lock – that is, a section of the river where the levels are lowered or raised between gates, to regulate traffic and water flow. J. comments on how irritating it is when women wear ‘boating clothes’ that are too delicate to get wet. George separates from the group to do some work for his employer in Shepperton. Harris proposes visiting a cemetery to see an interesting tombstone, but J. rejects this idea, finding cemeteries depressing. Harris falls into the food hamper while trying to get some whiskey.
When J. and Harris stop to lunch on the riverbank, a man accuses them of trespassing and tries to blackmail them. Harris, a large man, physically intimidates the visitor and they journey on. J. warns readers not to be taken in by these thugs, who usually do not work for the landowner they claim to represent.
He then recounts some embarrassing stories, in which he and Harris both make fools of themselves at pretentious parties – Harris by singing a comic song, and J. by pretending to speak German. J. describes a few more local points of interest, and the two men reunite with George in Shepperton.
Harris and J. convince George to tow the boat from the shore; towing is an arduous task that can lead to problems if the tower becomes distracted. J. recalls various incidents when he was boating and the tow-line became tangled or detached entirely.
The men have a satisfying dinner and sleep in the boat. The next morning, they wake up early and George tells J. a story about accidentally starting his day at 3 a.m. because he forgot to wind his watch. Later that morning, J. falls in the water and Harris fails in attempting to make scrambled eggs. As they pass Magna Charta Island, J. describes what it would have been like to be a peasant when the Magna Carta was signed.
When the men pass Datchet, they reminisce about an earlier trip, when all the inns were full there and they had to sleep at a local's house. That night, they sleep at an inn in Marlow. Montmorency chases a large cat, but is too intimidated to attack it.
The next day, they pass more historical landmarks, including Bisham Abbey. They run out of drinking water, and are disgusted when a local lock-keeper suggests that they drink from the river. Harris falls off the edge of a gulch while trying to eat supper.
The next evening, they cook Irish stew, and George plays the banjo. However, he is a beginner and his music is so awful that Harris and J. persuade him not to play for the rest of the trip. George and J. go for drinks in the town of Henley that night, but get lost on their way back. When they eventually find Harris sleeping in the boat, he explains that he had to move it because he was attacked by a flock of aggressive swans.
J. describes some of the mishaps that he and George experienced when they first learned to row. The men pass through Reading without incident, although J. does offer a brief history of the town. As they approach Goring, they discover a woman’s corpse floating in the water. They later learn that she drowned herself after having a child out of wedlock and finding herself unable to support it.
The men attempt to wash their clothes in the Thames, but the clothes only come out dirtier than before. That night, they drink at a pub in Wallingford with a large fish hanging on the wall. All of the patrons claim to have caught the fish themselves, but George accidentally knocks it over, and the men realize that it was made of plaster of Paris.
The friends continue toward Oxford, where they plan to turn around and row back toward London. J. describes a time that he and George went rowing and, by falling over at exactly the wrong moment, managed to ruin a professional photographer’s pictures. J. describes the attractions of Dorchester, Clifton, and Abingdon, which include Roman ruins and the grave of a man who fathered 197 children.
They manage to navigate a difficult stretch of river near Oxford, and spend two days there. J. interrupts the story to warn readers about renting a boat in Oxford because they tend to be of poor quality there.
On the way back from Oxford, it rains terribly, and the men find themselves cold, wet, and miserable. They soon decide to abandon the boat and spend the rest of the trip at an inn. That night, they enjoy a delicious supper and toast their decision to abandon the boat. Montmorency barks in agreement.