"I sleep with other women because I'm a poet, and poets feed off life," says Matthew Rhys as Dylan Thomas in John Maybury's four-cornered love story. That's a pretty glib statement, even from the not yet totally alcoholic young Thomas, and there's quite a lot of glibness in the film too.

It doesn't hesitate to embroider the truth for dramatic purposes, which is fair enough in literary love stories, and seems to be promoting as its central idea that female friendship is quite as important as and possibly longerlasting than heterosexual love. The friendship is between Caitlin, Thomas's put-upon wife, and Vera, his childhood sweetheart who became his on-off mistress.

The Edge of Love is at least a partial success, having an excellent period atmosphere and performances from a quartet of stars who do a fair job on a screenplay that moves backwards and forwards from the banal to the truthful as the plot progresses.

The foursome are Dylan himself (Rhys plays the poet as an overgrown schoolboy with an immense talent for poetry if not for life), Sienna Miller as Caitlin, Keira Knightley as Vera, the Welsh songstress who loved him but Dylan Thomas is torn between two women in a biopic that ends up as chaotic as the Welshman's love life formed a possibly lesbian-tinged friend-ship with Caitlin, and Cillian Murphy as William Killick. His romance with and marriage to Vera in the end caused a jealous shooting incident during which he sprayed the Thomas cottage with a Sten gun and was tried for attempted murder.

The film, written by Sharman Macdonald, Knightley's mother, starts nicely in a believable wartime London (though the sequence where Vera entertains those forced down the Tube by the Blitz is impossibly glossy). When Vera later meets Thomas in a pub, he is writing propaganda films. On the arrival of Caitlin, already his wife, the three become inseparable, eventually sharing a flat with Killick, who marries Vera before going off to war.

It is an uneasy foursome from the start, forged as much by alcohol, sex and sheer necessity as by anything much more profound. The problem with the film is that this lot are not a collection of human beings with whom one would like to associate - reading Thomas's poetry is a much more rewarding task than trying to be friends with him would have been. The foursome are all on the edge of tiresomeness rather than love half the time, and unable to control their own lives or the relationships they form with each other.

But perhaps this is Maybury's view too, and, assisted by strong performances from both Knightley and Miller, he shows that the strongest link was between the two women. Knightley is given the full visual treatment as a singer in the early scenes and responds surprisingly well. Miller, improving as an actress in each new film, makes Caitlin a believable figure as the wife who still feels for the woman who becomes her husband's lover.

In support, Rhys and Murphy do enough to make the film watchable, though if Thomas was as good-looking as Rhys the photographs we have of him fib.

Free spirits, perhaps, but also hopelessly lacking in sense, these four are thrown into marriage and parenthood during war without any real sense of what that means. At the end it all seems to be a huge muddle that can only turn out badly - rather like the film itself, which veers from glossy to grainy without reason but keeps one hoping for the best.

While The Edge of Love is nothing like as successful as Maybury's biography of Francis Bacon, Love Is the Devil, it shows Dylan Thomas to have been similar to the painter in his refusal to make compromises in either his life or his art, even if it meant behaving badly.