The novel details the lives of Tom and Maggie Tulliver, a brother and sister growing up on the River Floss near the village of St. Ogg's in England, probably in the 1820s after the Napoleonic Wars but before the Reform Act of 1832. Both the river and the village are fictional.
The novel spans a period of 10 to 15 years, from Tom’s and Maggie’s childhood up until their deaths in a flood on the Floss. The book is fictional autobiography in part, reflecting the disgrace that George Eliot (Mary Ann Evans) herself had while in a lengthy relationship with a married man, George Henry Lewes.
Maggie Tulliver holds the central role in the book. Her relationship with her older brother Tom, and her romantic relationships with Philip Wakem, a hunchbacked, sensitive, and intellectual friend, and with Stephen Guest, a vivacious young socialite in St. Ogg's and assumed fiancé of Maggie’s cousin Lucy Deane, constitute the most significant narrative threads.
Tom and Maggie have a close yet complex bond, which continues throughout the novel. Their relationship is coloured by Maggie's desire to recapture the unconditional love her father provides before his death. Tom’s pragmatic and reserved nature clashes with Maggie’s idealism and fervor for intellectual gains and experience. Various family crises, including bankruptcy, Mr. Tulliver’s rancorous relationship with Philip Wakem’s father, which results in the loss of the mill, and Mr. Tulliver’s untimely death, serve both to intensify Tom’s and Maggie’s differences and to highlight their love for each other. To help his father repay his debts, Tom leaves his desultory schooling to enter a life of business. He eventually finds a measure of success, restoring the family’s former estate. Meanwhile Maggie languishes in the impoverished Tulliver home, her intellectual aptitude wasted in her socially isolated state. She passes through a period of intense spirituality, during which she renounces the world, spurred by Thomas à Kempis’s The Imitation of Christ.
This renunciation is tested by a renewed friendship with Philip Wakem, with whom she had developed an affinity while he was a fellow pupil with Tom. Against the wishes of Tom and her father, who both despise the Wakems, Maggie secretly meets with Philip, and together they go for long walks through the woods. The relationship they forge is founded partially in Maggie’s heartfelt pity for broken and neglected human beings, as well as an outlet for her intellectual romantic desires. Philip’s and Maggie’s attraction is, in any case, inconsequential because of the family antipathy. Philip manages to coax a pledge of love from Maggie. When Tom discovers the relationship between the two, however, he forces his sister to renounce Philip, and with him her hopes of experiencing the broader, more cultured world he represents.
Several more years pass, during which Mr. Tulliver dies. Lucy Deane invites Maggie to come and stay with her and experience the life of cultured leisure that she enjoys. This includes long hours conversing and playing music with Lucy's suitor, Stephen Guest, a prominent St. Ogg’s resident. Stephen and Maggie, against their rational judgments, become attracted to each other. The complication is further compounded by Philip Wakem’s friendship with Lucy and Stephen; he and Maggie are reintroduced, and Philip’s love for her is rekindled, while Maggie, no longer isolated, enjoys the clandestine attentions of Stephen Guest, putting her past professions for Philip in question. In the event Lucy intrigues to throw Philip and Maggie together on a short rowing trip down the Floss, but when Stephen unwittingly takes a sick Philip’s place, and Maggie and Stephen find themselves floating down the river, negligent of the distance they have covered, he proposes they board a passing boat to the next substantial city, Mudport, and get married. Maggie struggles between her love for Stephen and her duties to Philip and Lucy, contracted as it were in her past, when she was poor and isolated, and dependent on either of them for what good her life contained. Upon arrival in Mudport she rejects Stephen and makes her way back to St. Ogg's, where she lives for a brief period as an outcast, Stephen having fled to Holland. Although she immediately goes to Tom for forgiveness and shelter, he roughly sends her away, telling her that she will never again be welcome under his roof. Both Lucy and Philip forgive her, she in a moving reunion, he in an eloquent letter.
Maggie’s brief exile ends when the river floods. The flood is considered by some to be a deus ex machina. Those who do not support this view cite the frequent references to flood as a foreshadowing which makes this natural occurrence less contrived. Having struggled through the waters