Mr. Tulliver owns a mill on the River Floss, and has a wife and two children. The children, Tom and Maggie, are different in appearance and character. Maggie is intelligent, wild, and dark; Tom is fair, inflexible, and has common sense rather than intellectual brilliance. Tom is sent away to be educated at the Reverend Stellings' -- there he meets a deformed boy named Philip Wakem. Lucy visits Tom and meets Philip there, too, and Philip forms an attachment for Lucy. Philip's father, Lawyer Wakem, is an enemy of Mr. Tulliver.
A series of unlucky events cause Mr. Tulliver to lose the mill which has been in his family for many generations. Mrs. Tulliver's sisters, all of whom are richer than the Tulliver family, refuse to help the Tullivers financially. The Tullivers must sell all their possessions and must leave their home. Tom, who has left school and is doing well in business, saves as much money as he can to obtain the mill back for his family. Maggie spends time with one of her cousins, Lucy, and enters into a flirtation with Stephen Guest, Lucy's fiance. Philip has continued to love Maggie, and Maggie has some kind of attachment for him, but Tom, who will always be angry at Lawyer Wakem, refuses to let Maggie see him. On the night that Tom is able to pay off his father's debts, Mr. Tulliver dies.
Maggie stays with various relatives, and, because she is resolved to be independent, decides to go away and become a governess. Stephen continues to make advances toward her, and they go on an ill-fated boat trip that causes Maggie to arrive back days later, with her reputation in ruins. She obtains a job as a governess, but she is unable to stay there because of the gossip. Tom, who is owns the mill now, refuses to let her stay with him because she has shamed the family.
Maggie plans to go away, but the Floss floods, and Maggie is swept downstream in a boat to the mill. It is flooded to the second floor. Maggie rescues Tom, and for a brief moment they are reconciled. They die in the flood, but at least are reuinted to their childhood closeness in death.
The characters are, in many ways, Victorian archetypes. Mrs. Tulliver, for example, is the picture of the upper-class lady with little brains and little interest in anything but material possessions. Her sisters are various incarnations of traditional Victorian women. Tom is the picture of the self-righteous, inflexible Victorian male. But Eliot gives certain characters remarkably unique characteristics -- such as Mr. Tulliver's often funny bluster and physical courage, and the touching deformity of Philip. Maggie became a template for later wild, headstrong heroines, since many of the heroines of novels before this had been of the yielding, traditional type. Tom is based, somewhat, on Eliot's own brother, and perhaps Maggie is a bit like the author.