Frankie Morrison (Jack McElhone) is moving -- again -- with his mother, Lizzie (Emily Mortimer), and grand-mother, Nell (Mary Riggans). Lizzie promises this is the last move, but, as Frankie observes, "she says that every time". As they are settling into a flat, we learn explicitly that Frankie is deaf.
We hear Frankie's voice over as he writes a letter to his father, Petty Officer Davey Morrison, care of his ship the Accra. It's clear Frankie is a frequent letter-writer, with a stamp collection supplied from his correspondent -- which, it turns out is his mother. Frankie's father is clearly not on the scene, and from Nell's reaction to a personal ad seeking Lizzie, it's clear the family is anxious to avoid being found.
At a new school (again) Frankie suffers some predictable petty bullying, and some counter-balancing friendship from another classmate, Catriona (Jayd Johnson). Frankie agrees to a bet with the chief pest, Ricky Munroe (Sean Brown), who asserts that Frankie's Da won't show up when the ship arrives.
Desperate as the day of the ship's arrival nears, Lizzie embarks on a pick-up outing to the local pub, but quickly leaves, disgusted and distraught. When Lizzie admits her predicament -- to find a total stranger to act as Frankie's Da for one day -- to her friend (and boss) Marie, Marie provides her a name and number. Lizzie meets the man, and when he agrees to the charade she hands him all the letters from Frankie to his Da.
Early the day the ship is due, Frankie bolts out of the flat and goes missing for hours. His surrogate Da is at the flat when Frankie returns. He gives Frankie a book that is somewhat skeptically received, until he tells Frankie that chose it based on what he read in Frankie's letters. Frankie's face lights up, and he runs to embrace the man.
At the football tryouts, Frankie delightedly claims the spoils of his bet from Ricky Munroe. The stranger and Frankie spend the rest of the day slowly getting acquainted, by the harbor. Frankie and the man ask Lizzie for one more day, which she initially rejects but is talked into.
The three spend a great afternoon together, and walking home Lizzie explains the background - that Frankie's Da had hit Frankie as a baby, which caused his deafness. The man and Lizzie show a sense of connection, illustrated by one sweet kiss good-bye.
Having finally responded to one of the ads searching for her, Lizzie meets her sister-in-law, and eventually visits her husband Davey in hospital, where he is dying. He wants to see Frankie, but when Lizzie refuses we see the anger and abuse that caused her to leave. She flees the hospital, though returns the next day with a photo and a painting from Frankie.
Davey dies, and Lizzie tells Frankie. She goes to stop payment on the PO box where she'd directed all Frankie's letters, and finds one last letter from Frankie... to the stranger, with an acknowledgement that his "real Da" has died. He hopes the stranger will visit them again.