Frankie Morrison (JACK McELHONE) is a 9-year-old deaf boy who's partially living in a fantasy world that's not of his doing. It seems that his mother Lizzie (EMILY MORTIMER) -- after fleeing her abusive husband, Davey (CAL MACANINCH) with a young Frankie and his grandmother Nell (MARY RIGGANS) -- has concocted an elaborate lie to explain the father's absence.
Ever since he was little, Lizzie has told him that Davey is a seaman who's constantly traveling around the world. Frankie is fixated on him and all things maritime related, checking out every book he can find and constantly writing to his father. The responses, however, are penned by Lizzie who intercepts the outgoing mail and writes back to the boy as his father. Nell doesn't approve of this deceit, but Lizzie believes she's protecting her son from a harmful and disappointing truth.
The family is always on the move, hoping to stay one step ahead of Davey, and have now arrived in Glasgow. Frankie is happy since they're by the sea, but Lizzie is horrified when she learns -- and realizes that Frankie knows -- that the ship she thought she had made up is actually arriving in port in a few days. Knowing they can't flee, Lizzie eventually decides to hire someone to pose as the boy's father for the few days he'd be in port.
When she can't find anyone, her boss, cafe owner Marie (SHARON SMALL), steps in delivers an unnamed stranger (GERARD BUTLER) who reluctantly agrees to the ruse in exchange for pay. Frankie's malcontent classmate Ricky Monroe (SEAN BROWN) bets him that his father won't show, but is shocked when "he" does. With the agreement being only to spend a day together, Frankie and his "dad" get in as much father/son time as they can, with Frankie showing a never before seen happiness and the stranger finding his dormant paternal feelings coming to a head.
As Lizzie hopes that the ruse will conclude without any revelations about the truth, her plan is put in jeopardy when her sister-in-law, Janet (ANNE MARIE TIMONEY), tracks her down and informs her that Davey is on his death bed and wants to make amends with his long-estranged family. From that point on, Lizzie must decide what to do and whether to continue her protective ruse with her son.