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Dylan Thomas may have been a great poet, but on the evidence of this film he was a pretty awful person. Of course, “The Edge of Love” suggests that he was surrounded by people just about as unpleasant as he was. That may all be true, but their story isn’t much fun to watch, especially since the picture’s combination of stylistic splash and pretentious turgidity makes for a long slog.

The film opens in 1940 London, where Thomas (Matthew Rhys) runs into an old girlfriend, Vera (Keira Knightley), who’s singing in the underground to the crowds herded there during German bomber attacks. Though she’s still attracted to him, she quickly develops a friendship with his emotional wife Caitlin (Sienna Miller) and in fact moves in with the couple.

The fourth member of the equation is a British officer, William Killick (Cillian Murphy), who encounters Vera on the street and is immediately drawn to her, despite an initial brush-off. Before long they marry, but he’s shortly shipped off to Greece. She moves next door to Dylan and Caitlin in neighboring rustic Welsh cottages, where she observes the couple’s mutual infidelities while raising the infant she’s given birth to. (The Thomases have a young child, too.)

A good deal of the picture is devoted to this trio’s supposedly unconventional lives together, but Sharman Macdonald’s script finds time to sketch William’s unhappy tour of duty in the ranks, after which he returns, deeply troubled, to England and takes up residence with Vera and his son. But the war has left him in emotional turmoil, and the plot takes a final turn into violence that leads to a trial in which Dylan’s character is tested and found wanting.

It’s difficult to know what Macdonald intends us to take from all this; it comes across is just another portrait of intellectual types suffering through their relationships. Though Rhys does a good job of capturing Thomas’ manner and speech, the script doesn’t give him a chance to reveal much of the poet’s inner life, and Miller’s strenuous effort to portray Caitlin as a tragically passionate and vulnerable character doesn’t really come off. Knightley (whose mother is Macdonald) seems mostly to be posing, and her singing is frankly mediocre; but she looks great in the slinky period clothes. As for Murphy, he’s an interesting actor, but his fragile frame and perpetually mournful attitude—even in the pre-Greece scenes—don’t seem quite right for William.

Visually “The Edge of Love” is odd, alternating prosaic scenes with others that strive—largely unsuccessfully—to have an arty feel, many of them distinctly undernourished in terms of lighting (though some blaze with a feverishly neon glow). The music score by Angelo Badalamenti is set against the old songs that Keira sings. The juxtaposition is intriguing but, like so much here, more peculiar than convincing.

One can admire the effort that went into a picture like “The Edge of Love,” but all the movie ultimately demonstrates is how difficult it is to make a persuasive autobiographical film about an artist. It may be accurate, but by the time it drags to an end you won’t really care whether it is or not.