When its focus shifts from two working-class boyhood pals to the precarious friendship one of them shares with a borderline psychotic stalking his sister, "A Room for Romeo Brass" detours from slice-of-childhood English comedy into something darker and much less entertaining.

A semi-autobiographical tale directed by Shane Meadows ("TwentyFourSeven") and co-written with Brian Fraser, his best friend for 20 years, the picture is ostensibly about a rocky patch in the relationship between a pair of once-inseparable neighbor boys.

In the beginning it plays like a life-affirming dramedy in which 12-year-old pranksters Romeo and Gavin (very natural newcomers Andrew Shim and Ben Marshall) escape their various woes and dysfunctional families through their mischievous friendship.

Gavin's row to hoe is some kind of degenerative bone disease that sees him occasionally laid up, sometimes for weeks at a time. Romeo has to cope with an absentee father who shows up from time to time, demanding angrily to be a part of his children's lives.

These are the kinds of stories English filmmakers have a unique talent for telling with deft touches of poignant humor.

But when Romeo becomes friends with Morell (Paddy Considine), a mentally unbalanced 20-something slacker, it becomes impossible for Meadows to maintain the light mood the film (and the audience) had settled in to. We recognize Morell as dangerously twisted, but Romeo doesn't catch on so quickly. He just thinks it's cool that a grown-up wants to hang around with him. It takes a while before the boy recognizes his new friend's hair-trigger temper and obsessive jones for his pretty, petulant sister (Vicky McClure).

The fact that Morell is clearly a ticking time bomb puts the kibosh on the comedy, especially after he goes ballistic in the wake of a practical joke the kids play on him.

Despite this discomforting change in tone, "Romeo Brass" stays engaging on one level because the performances of the mostly unknown cast are impeccable. Meadows likes tapping non-professionals to star in his films, and every character here feels 100-percent authentic.

However, the picture's mood swing amplifies the fact that the story is disjointed. Events and emotions are not clearly tied together. For instance, when Gavin is confined to his bed for a few weeks, Romeo never even visits him, and while it's a point of tension in the movie, his motives for neglecting his best friend are never adequately addressed.

Had Meadows toned down the psycho element -- either by reeling in Considine's alarming performance or having the character neutralized more quickly than he does -- "A Room for Romeo Brass" could have overcome these less significant shortcomings.

But as it is, the enjoyment of the movie is severely dampened by the looming storm cloud of this one intrusively unpleasant personality.