I guess Judd Apatow will only stop producing comedies when he has exhausted every single permutation of romantic mishap – and there are quite a few.

That's not entirely a bad thing, not when the results are of the quality of Bridesmaids and Knocked Up and when, as his band of comedy collaborators gets older, they allow that maturity to feed into the films.

The Five-Year Engagement is not about the romantic protagonists struggling to realise they love each other, or accepting the responsibility that comes with a relationship, or any of those faltering paths that we've seen many times.

It's about a couple who are richly in love and eager to do something about it, but are sidetracked by circumstance.

The story opens with San Francisco chef Tom (Jason Segel) proposing to his girlfriend Violet (Emily Blunt). There's jaunty humour in the proposal, but not the acceptance, which is immediate and unconditional. As they're planning the wedding, however, would-be academic Violet is offered a two-year university position in Michigan. Tom decides to put his career on hold and travel with her, and the pair postpone the wedding until their return.

But as Violet prospers under her dazzling psychology professor (Rhys Ifans) and is offered tenure, Tom finds that Michigan's modest culinary world has no need of his talents, and ends up baking bagels for the local deli. Suddenly the star chef is a faculty spouse, whose self-esteem plummets. It dawns on the couple that in waiting for the perfect moment to get married, they may have missed the boat altogether.

Director Nicholas Stoller has some decent form, with Forgetting Sarah Marshall and Get Him To The Greek. Here he lulls us into a false sense of familiarity, with an opening act of crude best friends and a cringe-inducing engagement party, before slowly introducing breadth to the comedy and depth to the emotions in play.

The humour comes from all directions, from Tom's decline into a bewhiskered geek to the psychologists' experiments with doughnuts, an uncensored pep talk from the parents to a shocking moment with a crossbow. Segel and Blunt expertly balance the comedy with the pain of the couple's dilemma and their individual faults – he falling into the martyr trap, she believing that selfishness is fine as long as you're honest about it.

Stoller and Segel, who co-wrote the script, avoid their own trap, of making Violet the villain of the piece; we're made well aware that Tom is suffering what too many women have before him.

The film gets a tad saggy in the final third; it may have benefited from being The Three-Year Engagement. But it's hard to be too strict when it has such smarts. "This is really exciting," Violet is told before everything goes pear-shaped. "This is your wedding. You only get a few of those."

We're going to be seeing Blunt again next week, in a less conventional comedy than this, Your Sister's Sister. The actress, whose flair for fun was first highlighted in The Devil Wears Prada, has the dramatic actress's versatility rather than the comedienne's fixed persona; she makes a strong, slightly strange impression as Violet, throwing in Lady Di and the Cookie Monster for good measure. It's hard not to adore her.