Jon Brittain’s Rotterdam premiered at Theatre 503 in 2015 to critical acclaim, and now gets a well-deserved transfer to Trafalgar Studios. Directed by Donnacadh O’Briain, the play introduces us to Alice and Fiona, an English couple living in the Dutch port of Rotterdam. After seven years together, Alice is finally ready to come out to her parents back home, when Fiona announces that she identifies as a man, and wants to begin living as Adrian. Which leaves a shell-shocked Alice questioning whether he’s still the same person she fell in love with – and if she stays with him, does that mean she’s now straight?

It’s a fascinating premise and forces both characters and audience to consider the labels we place on ourselves and others. But far from being heavy-going, Rotterdam is a warm and surprisingly funny play – expect to laugh, a lot, often at unexpected moments. Take tissues too, though, because it’s not by any means always an easy ride, and there are some incredibly powerful scenes in Act 2, as the impact of Adrian’s decision begins to be felt by both partners and those around them. And in the intimate setting of Trafalgar Studios, with the audience seated on three sides of Ellan Parry’s set, these emotions feel even more intense than they did first time around. With the actors only inches away – I was sitting so close to Alice as the play began that if I’d wanted to I could have read the email she was nervously drafting to her parents – Rotterdam feels less like a play and more like we’ve stumbled into the couple’s flat to intrude on some very private moments.

The original cast of four transfer with the production. Ed Eales-White provides comic relief, but also a voice of reason, as Alice’s affable ex Josh. It’s impossible not to like Josh, whose support is constant and unconditional, no matter what it costs him. Jessica Clark’s plain-speaking free spirit Lelani is great fun and more than a little bonkers, but with a touching vulnerability we only get to see in the play’s dying moments (and even then her exit line still gets one of the biggest laughs of the night).

But the show’s most powerful performances come from Alice McCarthy and Anna Martine. As Alice, McCarthy captures both the humour of the repressed Brit struggling to process emotions and experiences way out of her comfort zone, and the devastation of a lover whose world’s been turned upside down by forces out of her control. Anna Martine plays two roles in one, and her transition from Fiona to Adrian is exquisitely handled; as Alice herself points out at one point, her partner changes just enough to become someone different, but not enough for her to forget the person she knew. It’s a tricky balance to find, but Martine nails it and in doing so, manages to ensure we care just as much about Adrian in Act 2 as we did about Fiona in Act 1.

Aside from one scene in the pub that starts to feel a bit like a lesson in transgender terminology, Brittain doesn’t try to preach, or to tell us who’s right or wrong. Both Alice and Adrian have faults, and both at times handle the situation disastrously – but that’s far more believable than the alternative, and the play is all the more powerful for its honesty, however uncomfortable that honesty may be to watch.

Rotterdam is great entertainment, but it’s far more than that, of course; it’s the launch pad for an important discussion about the fluidity of gender and sexuality, and the nature of relationships in general – not just with lovers, but with friends and family too. (It’s particularly interesting to consider how the story might have been different had the characters been in the UK instead of far from home.) Stunning performances of a fantastic script make this a must-see production.