The complication is that Jane (Streep) was married to Jake (Baldwin) – but he left her and their kids for other woman Agness (Lake Bell). Now Jake’s on a nostalgia trip, sweet-talking his independent ex into bed and a whole world of confusion. This may sound like unforgivably caddish behaviour, but script and actor conspire to make Jake an attractive proposition, with Baldwin’s charismatic arrogance and playful banter in full effect.

Baldwin has terrific chemistry with Streep: the pair seem to be having a genuinely hilarious time getting drunk and naked together – not that Jane will do that with the lights on. The script explores female insecurities and doesn’t shy away from the realities of middle-aged sex: Jake proudly displays his hairy beer belly on several occasions.

You could watch this pair run around having a slapstick secret affair for hours. Unfortunately, Steve Martin has to poke his head in as the Sensible Alternative, and he’s miscast. Martin is fine in comic scenes, but as a romantic possibility he’s unappealing. The inevitable conflict between the two men takes the film into more humdrum, sentimental territory, not helped by periodic appearances from Jane’s soppy grown-up children. Still, Streep is as enjoyable as ever, and if laughs count for anything, this is one of the better romantic comedies of the season.

As ever, we are very firmly in a world of moneyed folk who don't think of themselves as such. F Scott Fitzgerald said that the rich are different from you and me. Meyers says the rich are exactly the same as you and me – because you're rich, right? Hey, me too! Anyway, even if you're not, these fictional rich people have adorable problems that you can sort of identify with, and you sort of feel rich by osmosis-empathy.

Streep plays Jane, whom I can only describe as a divorced chocolatier with grownup children and a thriving upscale deli-cum-eaterie. She's in the rich glow of her autumn years. Her ex-husband Jake, played by Baldwin, is a hotshot lawyer with sharp suits and a prosperous paunch who, to Jane's secret misery and humiliation, has remarried a sexy younger babe. This byotch actually left him briefly at some stage, had an obnoxious brat by some other guy, thus forfeiting our sympathy-rights, and is clingingly now back with Jake, killing their lust by demanding incessant trips to the fertility clinic. Just as Jane is on the verge of a relationship with a sweet architect called Adam, played by Steve Martin, Jake gets that old glint in his eye and now Jane's having a wild affair with her ex-husband! He looks longingly at her like Tony Soprano with a massive crush on Martha Stewart.

Now, all these people work for a living. In theory. They have the appurtenances and status-trappings of work. At one point, we see Meryl helping out at her restaurant behind the counter – and, phew! Isn't that hectic? Hard work, or what? We even see her implausibly picking vegetables in her colossal kitchen garden, wearing an outrageous "gardening hat". But basically these are the leisured classes we're talking about. Their skittish adventures are played out to the background music of the hotel bar pianist. Complicated? Well, money simplifies.

For me, Meyers's romcoms have been unwatchable because of the utter fakeness of their leading players: but I must confess, Baldwin and Streep do have that most over-analysed thing, "chemistry", and their marriage and sudden Indian summer of forbidden sex is weirdly believable – as unbelievable as that sounds. Not that there aren't annoying things in it. Meryl's new life-accepting-life-affirming laugh – her "Mamma Mia" laugh – is grating. But this is fun.