The Spanish director Nacho Vigalondo makes funny, fantastical, Frankenstein-like films that playfully combine small-scale with big-concept. His 2011 film "Extraterrestrial" is a romantic comedy centered on a handful of characters amid a massive unseen alien invasion. His "Timecrimes" was about a marriage filtered through a time-traveling murder mystery.

"Colossal," his second English-language feature and biggest production yet, fuses a traditional rom-com plot — big-city girl returns to her hometown — with a far more monstrous genre: the kaiju film. It's a tantalizing prospect. Who among us hasn't wondered what if Sally had met Godzilla instead of Harry? Would "Sex and the City" not have been improved had Mothra been on the loose?

In truth, "Colossal" is a more sly manipulation and inversion of genre. Gloria (Anne Hathaway) is an unemployed New York writer who spends her nights drinking before making apologetic early morning returns to her boyfriend's (Dan Stevens) luxury apartment. The more-together Tim, in the film's opening scene, has had enough. "I can't deal with you in that state," he says. He packs her bags.

Gloria retreats to her small-town home, crashing at her family's now empty house, and the movie starts taking the shape you'd expect it to. Gloria runs into an old friend, Oscar (Jason Sudeikis), who cheerfully hires her as a waitress at his bar. Gloria, again, doesn't make it to bed until the sun is up, spending nights drinking with Oscar and his pals (Tim Blake Nelson, Austin Stowell).

The mess Gloria — alcoholic and inconsiderate — makes turns out to harm not just those around her, but thousands of fleeing Koreans. She wakes to see news reports of a monster attack in Seoul. Later, she realizes with horror that the monster has her mannerisms (a particular way of scratching its head) and there's a strange coincidence between its regular appearances (always at 8:05 a.m.) and whenever Gloria steps onto a nearby playground.

To say more would risk spoiling the primary pleasure of "Colossal": watching Vigalondo juggle his outlandish premise with twists both realistic and implausible. There's a thrill to riding along with a movie that plays it straight-faced before so readily jumping into the absurd.

But it's a cheap thrill. "Colossal" sags under its high concept; its metaphors, not monsters, run amok. The movie's kaiju side is merely a fun-house mirror held up to its characters' emotional troubles, an eccentric mask for a fairly unimaginative story about a young woman trying to get her life under control.

The one-trick act of "Colossal" becomes tiresome even as its leads — particularly an excellent Hathaway — work to find some depth in the story. Most interesting is the turn that comes for Sudeikis' Oscar, whose old flame for Gloria is more sinister than you'd expect. This is the movie's more clever twist, but it feels less organic than it ought to — just a convenient way to lead up to the required monster melee climax.

Yet Vigalondo remains a tantalizing filmmaker who may well find a story to match his mash-ups. There's something in the way his characters' lives are refracted and manipulated through screens that resonates. (His last film, "Open Windows," was about a blogger lured into spying on his favorite actress through his laptop.) He revels in eradicating the chasm between us and what we watch.

"Colossal," a Neon release, is rated R by the Motion Picture Association of America for language. Running time: 110 minutes. Two stars out of four.

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