When The Rain Stops Falling, Almeida Theatre, London

Confusion rains in this gloomy epic

A fish falls from the sky. We'll have that fish for dinner in the last scene, along with a domestic reconciliation and a gathering of ancestral ghosts in hazy lighting. The rain is falling in London, Australia and Bangladesh. And there's a snow blizzard in the middle of the desert.

Andrew Bovell's play is an ambitious, meticulously worked out epic of family relationships across two continents and five decades, with a crucial coincidence in 1968, when a British businessman is convicted of child abuse in a London park and a boy is abducted by a molester on a beach in the Coorong, on the southern Australian coast.

But in its cross-fading of time and generations, the play becomes too much of a worry about which bit fits where. This is something you can do in the theatre more easily than in the cinema (where Bovell's script for Lantana starring Anthony LaPaglia was a conventionally chronological study of a disintegrating marriage), but it's still tricky.

And half the characters are called Gabriel, which doesn't help much. The son of the child abuser, Gabriel, has an affair with a waitress in the Coorong called Gabrielle and is promptly killed in a car crash. It's their son Gabriel who in 2039 catches the fish and cooks it for his son, who's not called Gabriel, but to whom he shows the postcards he received from the other Gabriel, his father. Blow Gabriel, you think, along with Ethel Merman.

You know something fishy is up (before it's down, that is) when the cast assembles in the pouring rain with umbrellas, discarding raincoats in a sombre ritual of silent disavowal. Every scene has a dutiful reference to the weather, and the fact that it could be worse. Then we have a scene at Ayers Rock where the bright red glow is replaced with a snowstorm.

The world is upside down in the same sort of way we always used to think Australia was anyway. My favourite bedtime game as a boy was digging for Australia, but having been there several times I dig it now for real. A similar thing happened to the director Michael Attenborough, whose experience of running a Shakespeare workshop in the painter Arthur Boyd's Bundanon retreat led to this collaboration with Bovell.

The Almeida cast includes two good Australian actors, Leah Purcell and Naomi Bentley, as the older and younger Gabrielle, as well as two outstanding British ones, Phoebe Nicholls and Lisa Dillon, as the first Gabriel's mother; the younger, Dillon version has a big speech about Diderot, which hangs there like a kite over Ayers Rock, along with the endless references to the planet Saturn and its child- devouring namesake.

It's this sense of heavy-handed compilation that undoes emotional involvement, as if Bovell is recounting a family saga in order to set his own record straight rather than spring any real dramatic surprises. The fish is a different sort of surprise, as is the consumption of Gabriel's ashes (which Gabriel's, you ask? I'm not sure) as a starter in the soup before the fish.

The play is given without an interval and runs for a bladder-bursting two-and-a-quarter hours, much longer than you want to sit still for unless it's Hamlet. And there isn't much in the way of light relief, unless you count the repetitive and significant remarks about rainfall across the world. Small talk about the weather is okay when it stays small.

But there is a beautiful lighting plot by Colin Grenfell, some fine rain, and some very fine acting from Tom Mison as the Gabriel who prangs the car, Jonathan Cullen as the child abuser – every family must have one these days in the theatre – and Simon Burke as a peripheral Joe called Joe who hitches up with the older Gabrielle.

To 4 July (020 7359 4404; www.almeida.co.uk)