Two teenage boys, Ellis and Neckbone, come across a mysterious man named Mud living in a boat lodged in a tree on an island in the Mississippi River. Mud is a fugitive with a troubled past, spinning wild tales of love and danger. He tells the boys that he's waiting for his true love, Juniper, and that he needs to evade bounty hunters who are hot on his trail. The boys are drawn to Mud's charisma and his romantic perspective on life. Sensing adventure and intrigued by the idea of helping him, they agree to assist Mud in his quest to reunite with Juniper and escape his pursuers.

As Ellis deals with his parents' failing marriage and his own burgeoning feelings for a girl from town, he sees in Mud a kindred spirit and clings to the hope of everlasting love that Mud represents. Neckbone, more skeptical, supports his friend but keeps an eye out for the potential danger their new acquaintance brings. Their small-town lives become intertwined with Mud's complicated existence, leading to a series of events that test their loyalty and courage. The boys learn hard truths about love, trust, and the harsh realities of growing up as they become involved in the adult world of conflict and sacrifice.