In the middle of a large river that flows from the mountains to the sea lies a small island. The river has not flooded the island in over twenty years. A mud-walled hut, built into a huge rock so that the rock forms its fourth wall, has stood on the island for ten years. Here live Sita, a young girl, and her grandparents. They keep goats and hens, tend a melon and vegetable patch, and live simply. In the center of the island grows a peepul tree, the island's only tree, old and sacred, which survived the last great flood. Grandfather is a skilled fisherman. His son, Sita's father, left years ago to work in a factory in a distant city. Sita is capable and strong but cannot read or write because neither grandparent is literate and the nearest school is across the river.
As the story opens, Grandmother has been sick with fever for three days. The monsoon rains arrived early, and now in mid-July the river is swollen and sounds more threatening than usual. Grandfather tells Sita that if Grandmother does not improve by morning, he will take her by boat to the hospital in Shahganj. He instructs Sita on what to do if the river rises: stay on high ground, retreat to the hut, and climb the peepul tree if the water reaches the hut. He plans to take the goats, as he may need to sell them to pay for treatment.
That night, heavy monsoon rain falls. The river's sound shifts to a moan and a hiss as loose earth crumbles into the current. Unable to sleep, Sita talks to Mumta, her rag doll and only companion of her own age, sharing her fears about Grandmother and the rising water.
At first light, Sita finds that the river has risen and turned muddy. Grandfather pushes off in his small dugout with Grandmother lying in the prow and three goats aboard. Grandmother, unable to speak, raises her hand in a blessing. The swift current carries the boat far downstream, and Sita watches until it shrinks to a speck.
Alone, Sita busies herself with chores. In the quiet hut she becomes sharply aware of her solitude, noticing Grandfather's hookah pipe in one corner and Grandmother's walking stick in another. She places Mumta within sight for comfort and recalls the stories Grandmother used to tell about the gods, particularly the playful young Krishna, wondering whether she will now have to discover those stories for herself. Stepping outside, she sees the river has spread far across the opposite plain. From the peepul tree she spots a drowned buffalo floating past, along with planks and a wooden bedstead, confirming that upstream villages are already flooded.
Back in the hut, Sita packs the family's valuables into a large tin trunk: the hookah, the walking stick, tins of food, and spare clothing. She begins cooking lentils but does not notice the water at her heels until it is oozing over the threshold. Panicked, she runs through ankle-deep water to the peepul tree and climbs up, forgetting Mumta in the hut. The island disappears rapidly beneath the flood. Only the big rock, the hut's roof, and the tree remain above water. A jet-black jungle crow settles in the upper branches near its nest, and its presence eases Sita's loneliness slightly. Then Mumta floats out of the hut and past the tree, carried downstream beyond reach.
The river continues to rise. Sita briefly glimpses a boat with people who seem to point toward her, but the current sweeps them away. The tree groans as the softening earth loosens its roots. At last the old tree tilts and moves into the main current, dragging its roots from the ground. The crow's nest tumbles into the water and the eggs are lost. Sita clings to a branch as the tree drifts, her feet in the water.
Exhausted, Sita hears someone calling. A small boat approaches, rowed by a boy who steadies himself against a branch and pulls her aboard. The boy introduces himself as Krishan. He explains that he comes from near the foothills and was trying to cross the river to report a badly flooded village when the current swept him downstream. They share mangoes from a basket under the seat, and the sweet fruit, Sita's first mango in over a year, momentarily makes her forget the flood. Krishan tells Sita he attends school; Sita says she cannot because there is too much work at home.
As evening falls, Krishan rows the boat into a flooded forest and ties it to a sturdy Toon tree, a type of Indian mahogany. Forest animals displaced by the flood pass through the water: a huge python glides by, and a large Sambur stag wades past. Sita sleeps while Krishan keeps watch. She wakes once and sees him sitting in the prow, looking blue in the moonlight like the god Krishna from Grandmother's stories. She dreams that Krishan and Krishna merge into one figure, and she rides beside him on a great white bird over the Himalayas into the realm of the gods. She wakes to the trumpeting of a lost young elephant calling for its mother. Unable to sleep, she listens as Krishan plays a flute, and its sweet music carries her back into dreams.
Morning reveals a clear sky. The flood has spent itself. They row through the flooded forest and emerge into shallow water over fields. They reach a hilltop village populated by Jats, members of a farming community of northern India, who give the children food and shelter. Sita is anxious to find her grandparents, and an old farmer offers to take her to Shahganj. Krishan decides to stay to wait for his own people. As Sita climbs into the farmer's bullock cart, Krishan hands her his flute, asking her to keep it for him and promising to come for it one day.
The journey takes a day and a night. In the bazaar of Shahganj, Grandfather does not recognize Sita until she charges at him and clasps him around the waist. Before she can ask about Grandmother, the dazed look in his eyes tells her everything: Grandmother has died. Sita wants to cry, not for Grandmother, whose suffering is over, but for Grandfather, who looks helpless and bewildered. She forces back her tears, takes his trembling hand, and leads him down the crowded street, knowing it will be her shoulder he leans on in the years to come.
When the river subsides, they return to the island. Grandfather is astonished to find the peepul tree gone. He says the tree saved Sita. "And the boy," Sita adds. They rebuild the mud hut against the big rock. The trunk Sita packed survived, though the water ruined most of its contents; Grandfather's hookah is saved, and in the evenings he smokes it with some of his old contentment, telling Sita stories of floods he experienced as a boy. Sita plants a mango seed where the peepul tree once stood, imagining sitting in its branches someday. Grandfather plants peas, carrots, gram, and mustard.
Sita tries to play Krishan's flute at the water's edge but can only produce broken notes. The rains end and life returns to normal along the river. One day, sitting on a rock at the island's far end, Sita finds a small painted wooden peacock washed ashore, a rare toy for a girl who has none. She hears flute music, closer than ever, and feels a soft footfall in the sand. Looking up, she sees Krishan. He promises to come more often and offers to teach her to play. They sit together cooling their feet in the now-clear water. Sita reflects that the river is sometimes angry and sometimes kind. Krishan replies that they are part of the river and cannot live without it. The narrative closes by echoing its opening: "It was a good river, deep and strong, beginning in the mountains and ending in the sea." Along its banks live millions of people, and Sita is only one small girl among them, known to no one except the old man, the boy, and the river.