Desiree Leanne ("Desi") Gill was four years old when her father died. Now, a year later, she is still struggling to accept her father's absence. Her mother Rhonda (Samantha Mathis), having just relocated both herself and her daughter to live with her parents, is concerned that Desi does not understand what has happened, as she still asks where her father is and only refers to herself in the third person.
Desi's grandmother Trish (Ellen Burstyn) has the idea of helping Desi write a letter to her father. They leave the letter at her father's grave, and every day Desi waits for the postman to bring her a reply. However, when the letter is returned to them, Desi's emotional and mental state worsen. Desperate, Desi and her mother go to the carnival with a local pilot, where Desi is given a balloon with a picture of a mermaid on it. Desi's grandmother has the idea of attaching a letter to the balloon and sending it to Heaven.
4000 miles away, a man duck hunting on the Mermaid River outside the town of Mermaid finds the balloon. Touched by Desi's letter, he and his wife reply. Upon receiving their letter, Desi begins to recover and the two families form a bond.