Filled with improbabilities and weighed down by stilted, heavy-handed narration, the “Masterpiece Classic” production “Small Island” ultimately overcomes both, thanks largely to the strength of its cast and underlying power of its story. Shamelessly melodramatic and old-fashioned, this story of racial politics, love and sacrifice in England during World War II and its aftermath ought to feel fresh to an American audience, provided viewers can get past the thick fog of schmaltz to appreciate what lies beneath.

Adapted from Andrea Levy’s novel, the story explores a central quartet of characters in parallel and eventually overlapping storylines — beginning with Hortense (Naomie Harris, put to much better use here than in the “Pirates of the Caribbean” films), a Jamaican woman who dreams of immigrating to London.

Raised by an adoptive family, she secretly longs for her “brother” Michael (Ashley Walters), who doesn’t share her ardor — and soon leaves himself to defend the “mother country” by flying in the RAF. There, he becomes involved with the unhappily married Queenie (“The Prisoner’s” Ruth Wilson), whose awkward, inept husband Bernard (Benedict Cumberbatch) has also gone off to war — and turned up missing.

Hortense seizes on a plan to escape Jamaica by marrying another islander-turned-soldier, Gilbert (David Oyelowo). As luck or fate would have it, Gilbert meets Queenie and winds up renting a room from her. Eventually, these various threads are destined to coalesce, with the explosive mix of interracial romance, infidelity and longing creating a rather combustible brew.

Directed by John Alexander from a screenplay by Paula Milne and Sarah Williams, “Island” contains elements every bit as sappy as a Harlequin romance novel. Lines of narration like, “The pain of lost dreams is too much for any human soul to bear,” or about love leaving “a footprint on your heart” are teeth-gnashing, and the gauzy, soft-lit sex scenes play as dated even within the period setting — as does the title’s too-cute play on words.

The two-part production, however, is buoyed by powerful performances from the two key women, Harris and Wilson, as well as Oyelowo’s inherent decency and the scenes with Cumberbatch (also splendid in BBC America’s “The Last Enemy”), who brings an awkwardness to Bernard that manages to render him oddly sympathetic despite his assorted flaws, not the least being a bigot.

For an American audience, “Island” also explores questions of race in a novel setting, one where the Jamaicans — having just fought to protect their new home — are still subject to discrimination and derision. It’s like holding a mirror, in a way, up to the U.S.’ own painful past.

During part one especially, “Small Island” feels like something of a stretch for the “Masterpiece” franchise. Before it’s over, though, this handsome project earns its place there.

Small Island

Two-part movie; PBS,Sun. April 18-25, 9 p.m.

  • Production: Filmed in Northern Ireland by Ruby Television in association with AL Films for the BBC and WGBH Boston. Executive producers, Rebecca Eaton, Alison Owen, Paula Milne, Paul Trijbits, Lucy Richter; producers, Joanna Anderson, Vicky Licorish, Grainne Marmion; director, John Alexander; writers, Milne, Sarah Williams, based on the novel by Andrea Levy;
  • Crew: camera, Tony Miller; production designer, Will Hughes Jones; editor, Roy Sharman; music, Martin Phipps; casting, Nina Gold, Robert Sterne. 3 HOURS.
  • Cast: Hortense - Naomie Harris Gilbert - David Oyelowo Queenie - Ruth Wilson Bernard - Benedict Cumberbatch Michael - Ashley Walters Narrator - Hugh Quarshie