“About Time” opens Friday at Point, Eastgate and Star Cinema. R, 2:03, three stars out of four.

The moment of truth comes early on in writer-director Richard Curtis’ “About Time.” Is he going to be able to sell the audience on the film’s central conceit, that a father (Bill Nighy) and son (Domhnall Gleeson) can travel through time?

Luckily, he has the wonderfully dry Nighy in his arsenal who, in explaining the family trait to his son TIm after he turns 21, stammers and winces his way through the explanation, half-embarrassed even. Clenching your fists and going to revisit any moment in your past? It just seems so . . . un-British, Nighy seems to convey.

Sold — at least for the length of Curtis’ charmingly slight film. Curtis has always used acerbic, melancholy Brits to sell his romantic blancmanges (Hugh Grant, Colin Firth, Bill Nighy) and “About Time” continues the streak. It’s sweet and charming and utterly preposterous and, despite the trailers, it’s not as much of a love story as you might think. Or at least, not that kind of love story.

The ground rules for Tim are that he can revisit at any moment in his past simply by going into a dark place and clenching his fists (were this true of everyone, I would have time-traveled as a teenager every time I went on break at McDonald’s). Stepping into the shoes of his earlier self, he could conceivably make a killing in the stock market, or fly to New York and stop 9/11.

But this is a film by the guy who made “Love Actually,” so Tim uses his powers to fall in love. He meets cute with an American book editor named Mary (Rachel McAdams), but then blows it when he goes back in time to save his grouchy roommate’s (Tom Hollander) play, which then erases the meet-cute timeline. So he has to backtrack twice more to find Mary again and reverse-engineer a different meet-cute. It’s like “Looper” with more Ben Folds songs on the soundtrack.

After all that machination to get Tim and Mary together, it’s somewhat surprising to see “About Time” sail blithely on past that rush of first love, as Tim and Mary get married, buy a house and start having kids. (I actually liked their relationship better here, which has a lived-in, teasing affection.) The momentum of the film starts to flag in the middle (Curtis the director has always been a generous soul to Curtis the writer), as we wonder where this is going.

But then Dad drops a new time-travel rule on Tim — once you have a child, you can’t go back to a point in time before that child is conceived. It’s a plot point that serves as a neat dramatic metaphor for how parenthood so fundamentally changes you, and sets in motion the tearjerking third act.

You see, the central couple in the film isn’t really Tim and Mary — it’s Tim and his Dad. The scenes between Gleeson and Nighy — playing table tennis, reading Dickens, exchanging very un-British exchanges of affection — are the best in the film. Curtis succumbs to a sort of enjoy-every-sandwich philosophy in the film, but it works because he surrounds what could be treacly sentiment with so many grace notes, such as Lindsay Duncan as Tim’s flinty mother, or that Nighy’s character used his primary time-travel gift to get more reading done. (“Books, books, books,” he enthuses.)

Curtis has said “About Time” is the last movie he’ll direct, and he may be better off writing more screenplays that other directors can sharpen and hone to a fine point. But “About Time” is a sweet comedy-drama that’s worth living through — at least once or twice.