I and You, Hampstead Theatre review - Young Adult drama packs emotional punch

Here's a good pub quiz question: after Shakespeare, who was the most performed playwright in America last year? Arthur Miller? Tennessee Williams? David Mamet? None of those.

We are in a teenage bedroom – messy, with lots of pinks, fairy lights and glitter. There's a slightly claustrophobic air about it which makes sense when we learn that its occupant, Caroline (Maisie Williams, Arya Stark in Game of Thrones), is all but confined there while she awaits a liver transplant. She is reliant on her mobile phone, to which she is almost surgically attached, for most of her contact with the outside world. Visiting her is a classmate, Anthony (Zach Wyatt), whom she barely knows.

Caroline is full of sassy one-liners - 'I'm small but mighty, like a dachshund'

He is working on a school project on Walt Whitman – beguilingly, he's having problems with the poet's use of pronouns in Song of Myself, which gets a starring role here – and asks for her help with the assignment, which he has to hand in the next morning. Over the course of an evening they bicker and flirt and, prompted by the poem, discuss the meaning of life, fear of death and the differences between boys and girls.

Caroline is full of smarts and sassy one-liners – “I'm small but mighty, like a dachshund.” He is a geeky, basketball-playing jazz fan whose interests are in stark contrast to her love of Elvis Presley and cats, and he says he doesn't “get girls because they get all weird and then they get mad”. They are very believable characters.

We see Caroline, who has hardened herself to the possibility of an early death and who rejects people being “nice” just because she's an invalid, gradually become emotionally alive again, as Anthony struggles with not just what Whitman's words mean, but also the meaning of life itself. As she says sardonically, that's the problem with trying to understand poetry, because when you think it means one thing, it turns that “it means 14 other things.”

Williams and Wyatt give engaging and accomplished performances. He nicely captures the teenage boy's mix of awkwardness and assuredness, while she can switch from bolshiness to vulnerability in a beat. Hall directs with great subtlety, while Michael Pavelka's design has great flourish, and the clever twist at the end packs a real emotional punch.

• I and You at Hampstead Theatre until 24 November
• Read more theatre reviews on The Arts Desk