Not quite a perfect 10. But still, Scottish director Eva Riley smashes it with her debut feature, a gritty and tender portrait of a teenage gymnast who meets her half-brother for the first time. It’s a drama in the social-realist tradition of Ken Loach and Andrea Arnold, with whom Riley shares an instinct for working with first-time actors. Her young stars – Frankie Box, a real-life gymnast, and Alfie Deegan, a trainee joiner – are naturals. Their real and raw performances give the film warmth and energy. It had me hooked from the opening scene.

Box is Leigh, a stroppy 14-year-old who lives outside Brighton. She isn’t the best gymnast on her squad, but she trains the hardest – though since her mum died a year or so ago, she’s lost her confidence on the mat. The other girls bully her for wearing tatty old leotards and not having enough money to pay subs. Her dad is rubbish; his strategy for single fatherhood seems to be avoidance. Then one night Leigh comes home to find a stranger in the front room: Joe (Deegan), her dad’s son from a previous relationship (though they’re so close in age, you suspect there may have been a crossover).

The chemistry between the two actors crackles. Joe nicks motorbikes for a local gang. He’s a bit of a geezer, but Deegan’s heartbreakingly open performance shows how vulnerable he is, and with a huge capacity for love that doesn’t seem to have been met by anyone – not by his dad, that’s for sure. There’s lump-in-the-throat scene where he tucks Leigh into bed.

Soon Leigh is hanging around with Joe’s mates. Her fearlessness and physical strength from gymnastics make her a natural at thieving. Riley cranks up the tension as Leigh crosses into this dangerous and thrillingly sexy world of gangs and boys, her sense of self in the balance. I watched a couple of scenes behind my hands, willing nothing bad to happen to her. What an emotional, satisfying film this is – and a whopping oversized calling card for everyone involved.

• Perfect 10 is available on digital platforms.