Kaadu unfolds deep within a forest where tribal communities live steeped in rigid superstition and unquestioned belief in fate. A newly appointed forest officer Vasu (played by Sathaar) arrives determined to protect the forest, enforce order and bring progressive change among the tribal communities living deep within the woods. Courageous and upright, he earns admiration after rescuing his senior officer's daughter Asha (played by Reena) from a wild elephant, and she soon falls in love with him.

To assist him in his remote posting, a young tribal woman Chella (played by Vanitha) joins his household. Reminding him of his deceased sister, he develops a protective, brotherly affection for her. Courageous in duty, Vasu cracks down on wood smuggling operations and risks his life safeguarding the forest's wealth. At the same time, he reaches out compassionately to the tribes—arranging medical aid during outbreaks of illness and urging them to abandon dangerous rituals.

The conflict escalates when extremist elements within the tribe decide to offer Chella as a human sacrifice to appease their deity. Standing alone against centuries of blind belief, Vasu chooses to protect her at any cost. In a final act of courage, he sacrifices his own life—turning his death into a searing indictment of superstition and a plea for rational thought.