Starring Rosamund Pike and Billy Connolly. Rated G.

This blandly titled comedy from the U.K. spends so much setup time being generically clever, it’s truly unexpected when, a third of the way in, things heat up in oddball ways.

Gone Girl Rosamund Pike and Doctor Who veteran David Tennant play Abi and Doug, middle-class Londoners on the edge of divorce, forced to get along during a trip to Scotland, for the birthday of Doug’s dad. Fortunately, granddad Gordie is played by Billy Connolly, wearing a twinkle, granny glasses, and long white hair. Gordie’s party seems ready to be overshadowed by the self-serving grandiosity of Doug’s stuffy brother (tube stalwart Ben Miller) and the man’s mousy wife (Amelia Bullmore). Then there’s the messy detail of Gordie’s long struggle with cancer—something made more poignant with Connolly’s own recent Parkinson’s diagnosis.

The battles between, and within, the main family units are pretty dull, if gamely performed, and Anglo familiars like Celia Imrie and Annette Crosbie help prop up the later, flatter scenes. But as so often happens in real life, this vacation is really for the children.

Cowriters and directors Andy Hamilton and Guy Jenkin worked on such Brit-TV staples as Not the Nine O’Clock News and Outnumbered. The latter series must have taught them how to handle small actors, because the kids here are phenomenal.

Experienced Emilia Jones plays Doug and Abi’s eldest, a tween worrywart who takes notes to keep current arguments straight; little Bobby Smalldridge is the middle boy, a sweet-faced dreamer unable to lie; and volatile newcomer Harriet Turnbull plays a mouthy preschooler who could give most barristers a run for their powdered wigs. Once Gordie gets the children to himself, in waterside locations that turn dour Scotland into a sunny vacation paradise, we’re given one funny, heartbreaking, and expertly judged twist after another.

For some reason, the movie struggles to return to sitcom whatevs and dropped plot threads for a forced happy ending. But it’s the bittersweet stuff you’ll remember most fondly when this Holiday is over.