A working-class heartwarmer

“Dear Frankie” stars Emily Mortimer as a single mother who creates a make-believe dad to compensate for the one her 9-year-old son has already got. Lizzie (Mortimer), Frankie (Jack McElhone) and her mother, Nell (Mary Riggans), have been running from the abusive Davey (Cal Macaninch) for years, but Lizzie has been telling Frankie that his father is a sailor on board a ship called the Accra. And Frankie has been plotting his “da’s” course on the world map above his bed for years, according to the stamps on the letters he regularly receives from him. What Frankie doesn’t know, or doesn’t let on if he does, is that the letters are written by Lizzie, who explains away their Scottish postmarks by saying they pass through a mail depot in Glasgow.

Nell wants Lizzie to tell Frankie the truth, especially because the actual Accra will be docking at Greenock, where they live, in a few days. Frankie has bet a schoolmate his entire international stamp collection that his father will make it to his football tryouts, and a desperate Lizzie enlists her friend Marie (Sharon Small) to help her find a man willing to pose as the boy’s father for a day. Marie comes through, and one day Frankie finds a handsome, if somewhat uncomfortable, stranger bearing a colorful book on marine life sitting in his kitchen.

With her lank brown hair, sad eyes and rangy frame, Mortimer is a natural for this kind of role -- the tough but vulnerable single mom perennially clad in boiled wool. She is so convincing in the part that it’s easy to forget how adept she is at transformation (she played a glamour-girl socialite in “Bright Young Things” and an insecure actress in “Lovely and Amazing”). Mortimer is capable of a quick, secret intimacy that makes her especially believable as the single mother of a deaf child with whom she communicates with her own mix of sign language and gestures. Riggans’ Nell is by far the saltier of the two -- she smokes and carps and worries about the dream world Lizzie has created for Frankie. In some lights she looks a little like Anne Meara, and you half expect her to say something horribly inappropriate at just the right time.

Directed by Shona Auerbach and written by Andrea Gibb, “Dear Frankie” nestles comfortably in that Scottish-Celtic niche of cozy, overcast, working-class fairy tales that seem to smell faintly of fried fish and beer. Gerard Butler, who was young Christine’s aristo-lover in “The Phantom of the Opera,” plays the stranger who pretends to be Frankie’s dad, though he’s appropriately scuffed and stubbly-faced here. Despite his salty-dog appearance (he looks as if he walked out of a J. Crew catalog), the man is about as hard-boiled as a marshmallow -- within 12 hours he’s wondering out loud how Frankie’s father could have left the two of them and lobbying hard for a second day en famille.

Not that “Dear Frankie” aspires to any kind of hardened realism. On the contrary, it caters to a particular type of Anglophile fantasy, the kind where the china doesn’t match and the chintz is dingy, but people look out for one another and love sprouts easily in the humidity. As a boy cut off, to some degree, from the world around him, McElhone is appealingly grounded and projects a certain degree of self-sufficiency that allows him to engage in his mother’s fantasy without threat of too much outside disruption. “Dear Frankie’s” surprises are few and low-key, but the story wraps up nicely. In that way, the movie is not unlike the fish dinners Frankie (who doesn’t eat fish, out of some sort of solidarity with marine life) procures from Marie -- slightly soggy and bland, but as warm, starchy and satisfying as a box of fries.

*

‘Dear Frankie’

MPAA rating: PG-13 for language

Times guidelines: Family fun

Emily Mortimer...Lizzie

Gerard Butler...Stranger

Sharon Small...Marie

Jack McElhone...Frankie

Mary Riggans...Nell

Miramax Films and Pathe Pictures present, in association with the UK Film Council and Scottish Screen, a Scorpio Films production, in association with Sigma Films, produced in association with Inside Track Productions, released by Miramax. Executive producers Stephen Evans, Angus Finney, Francois Ivernel, Cameron McCracken, Duncan Reid. Producer Caroline Wood. Director Shona Auerbach. Screenplay by Andrea Gibb. Director of photography Shona Auerbach. Editor Oral Norrie Ottey. Production design Jennifer Kernke. Music Alex Heffes. Running time: 1 hour, 45 minutes.

At the ArcLight, 6360 Sunset Blvd., Hollywood, (323) 464-4226; and Landmark’s NuWilshire, 1314 Wilshire Blvd., Santa Monica, (310) 281-8223.