It’s strange how committed and exact theatre can communicate uncertainty and lack of conviction. Faslane may start with a manifesto and end up in a “No to Trident” demonstration, but in between there’s Jenna Watt standing in a rockpool by the Gare Loch, looking over to the submarine base where her uncle works, trying to make up her mind about the UK nuclear deterrent while thinking of cake and Forrest Gump.

Perhaps it was inevitable that the first of the Royal Navy's nuclear submarines – launched in 1966 – was called HMS Resolution. Did they hope to reassure us that all the fuss was over? There’s not a placard in sight in Faslane, just a wee CND badge, but there’s a banging sound and roving mics to amplify all that’s threatening and doubtful.

Jenna is Scottish – that’s important – and just old enough to have been dead sure that the Spice Girls were huge. Not as huge as a Vanguard class submarine though, which she knows all about and more. She’s happy to talk about it: to tell us about Trident missiles, where they’re stored, their twelve independently targetable warheads, and what they could do. She’s also listened a lot to the ongoing debate.

You’ll hear voices from the time before Geri Halliwell (that’s prehistory for Jenna), up to today with Prime Ministers Cameron and May, and First Minister Nicola Sturgeon. She knows about the UN too. But Jenna makes it personal, visiting the Peace Camp, making friends there, and talking with her uncle, whose job on the ‘nukes’ is primarily to make them safe. He has, of course, signed the Official Secrets Act and she’s ok with that, zipping her mouth ruefully.

Faslane is a very contemporary performance, politically engaged, but on friendly, colloquial terms. Watt is in her bare feet (for the rock pool) and runs from side to side of the stage with her mic to report divided opinion. A soundscape from Kim Moore conjures quiet white noise, an engine pulse, and the pounding beat of a dramatic alert.

As Watt minds her footing on the rocks, so Faslane carefully steps between anecdote, commentary and theatre. When it slips, which happens once or twice, it’s because the tone gets a little arch and cutely mischievous. There’s one moment at the end that seemed too obvious and felt unnecessary. But as it happens, I have been on board a laid-up Resolution class nuclear sub and I well remember the idle chat as we walked past the missile tubes; if nothing else, Faslane makes that impertinence impossible.