In Arkansas, 14-year-old Ellis lives in a floating house with his mother Mary Lee and his father Senior. Ellis helps his father selling fish with his best friend Neckbone. Ellis and Neckbone find a motorboat stranded on the top of a tree in an island on the Mississippi River since a flood and they plan to fix the boat for them. One day, they climb the tree and they find fresh meal inside the boat meaning that someone is living in "their" boat. They leave the place and meet a man named Mud near their boat. Soon they learn that the passionate Mud killed a man that had beaten up his girlfriend Juniper and now the father and the brother of the deceased man are hunting him down. Mud makes a deal with the romantic Ellis and Neckbone to help him to reunite with Juniper. But when Ellis meets Mud's stepfather Tom, he has a different opinion about the love of Juniper with Mud.