Nine-year-old Frankie and his single mum Lizzie have been on the move ever since Frankie can remember, most recently arriving in a seaside Scottish town. Lizzie has invented a story which he is away at sea on the HMS Accra. Every couple weeks, Lizzie writes Frankie a make-believe letter from his father, telling of his experiences in exotic lands. As Frankie tracks the ship's progress across the world, he finds that it is due to dock in his hometown. With the real HMS Accra arriving in only a fortnight, Lizzie must choose between telling Frankie the truth or finding a perfect stranger to play Frankie's father for just one day...