Tracey Emin, who once told the story of fleeing the stage during a disco-dancing competition as teenager in Margate after men shouted “slag” at her, has got to be an inspiration for this impressive zero-budget social-realist drama set in Margate that features a similar act of cruelty against its teenage heroine. The film begins in kitchen-sink-grim mode, then multiplies in interest as it transforms into the origins story of a female talent – here in standup comedy rather than art. It’s a striking directorial debut by James Gardner, and has an astonishingly good performance by Liv Hill as Sarah, a 15-year-old written off by teachers as lazy and anti-social.

Sarah is coping with the responsibilities of looking after her mentally ill mum and younger siblings. Homework ranks on her priority list somewhere below finding a fiver for the electricity meter. She is also holding down a part-time job at an amusement arcade where she gives handjobs to sad old blokes for a tenner. Her drama teacher (played by Cyril Nri) spots the comedy potential in Sarah’s sarky putdowns – this to her mum, who promises vaguely to get a job: “They wouldn’t employ you as a speed bump” – and gives her a spot at the school’s talent show.

The subject matter in Jellyfish is bleak, but by letting Sarah tell her story as she finds her voice as a comedian, it doesn’t exploit her vulnerability. Sarah’s comedy superpower is saying the unsayable. Despite occasional stumbles – a few too many jabs at gentrification, a couple of misfiring performances, and in places the budget constraints really show – Jellyfish is a calling card for everyone involved, especially Hill, who always shows the flickers of Sarah’s inner life, even as her character feigns blank, not bovvered.