Orphaned at a young age, Kali and his sister Valli survived by performing on the streets—Kali on the drums, Valli perched atop a pole balanced by an older boy. As adults, the siblings live in a hilltop village where Kali works for a powerhouse in the valley below, operating a winch from a shed that ferries workers to and from the tunnel below. The powerhouse’s new divisional engineer, the rule-abiding but fair Kumaran, notices Kali offering free rides to unauthorized townspeople. When he orders the passengers out, citing rules of operation, Kali becomes angry and stops the trolley halfway downhill. Kumaran shrugs off this act with a knowing smile, signaling a tension that will simmer between them.
One day, while near the canal, Valli encounters a young woman named Mangaa, who, along with her mother, has arrived from a drought-stricken town, Ilaiyangudi. The mother is searching for work, and both women are hungry; Manga reveals a ravenous appetite for fish, which quickly becomes a defining trait. Valli feeds and shelters them for the night, and asks Kali about allowing them to use the vacant house next door. Meanwhile, Kali roughs up his fellow employees whom he suspects of snitching on him to Kumaran. Kali is given a final warning by Kumaran as a prelude to serious disciplinary action.
While Kali is at work, Manga appears, picks up his watch and teasingly declares she will run away with it. They chase each other into the woods and end up waist-deep in the river, where Kali finally retrieves the watch and rushes back to the shed. During his absence, a child living in the valley has needed medical attention. After the emergency alarm at the shed has rung for several minutes, a passerby steps in to operate the winch. The child is transported back in the trolley and dispatched to a hospital. The following day, Kumaran suspends Kali from his job for ten days for gross misconduct.
That evening, Kali goes to Mullimalai to assuage his anger at Kumaran among the Badagas, joining them in singing and dancing. While staggering back home drunk, Kali passes out on an unlit street. A truck drives over his left arm, which is amputated at a city hospital. As a result, Kumaran regretfully fires Kali since he can no longer perform his job adequately with one arm. At Valli's prompting, Kali and Manga agree to marry; they have a simple wedding ceremony held at the local temple. Kumaran attends and brings a present of cash, which Kali rejects, but Manga accepts.
Kumaran becomes attracted to Valli. After determining her willingness for marriage, he requests permission from Kali who asks him to return the following day. Later that day, Kali corners Murugesan, a philandering local grocer, and offers him his sister's hand in marriage. An engagement ceremony for Murugesan and Valli is arranged. At the ceremony, Manga angrily interferes in the main ritual, which makes Kali furious at his wife. He physically assaults her repeatedly, but Manga remains firm in her opposition.
The next day, Manga convinces Kumaran and Valli to marry without Kali's knowledge, but he accosts their wedding party on its way to the temple. Kali accuses his sister of abandoning him because of his handicap. Valli, overwhelmed with guilt, rushes into his arm. Kali holds her close, declaring that no matter what, he remains the most important man in her life. Though he still harbours resentment toward Kumaran, Kali's pride is restored. He offers his blessing.
