With a title like Humphrey Ker is Dymock Watson: Nazi Smasher!, you might imagine that his show is a Boche-bashing, Boy’s Own-style romp in which our hero takes on Hitler’s war machine. And you’d be right. But it’s also based on the real-life exploits of Ker’s maternal grandfather who, as part of the secretive Special Operations Executive, parachuted into enemy-held Romania in 1943 to co-ordinate with local resistance and destroy oil pipelines.

‘Nobody in my family knew about his story until he died, until there were obituaries in The Times and The Western Mail,’ Ker explains. ‘My mum was astonished.’ Often working alone, SOE agents had poor survival prospects. Watson’s file is locked under the Official Secrets Act until 2032, so Ker – the Dreadfuls’ resident history buff – garnished his research with a certain amount of artistic licence, reassuring his concerned mother that ‘no, I don’t believe he wrestled a Nazi on top of a dam. But I do think it will make for a better show’.

Watson’s magic skills are an invention as well, but the brutal reality is that his grandfather was sent ‘to kill’, acknowledges Ker. ‘And it doesn’t matter how much you make out that the Nazis were robots, they were people too. The subject is close to my heart and I’m determined not to make comedy capital out of a war in which so many did appalling things and so many sacrificed their lives. My grandfather was a hero and reminds me that I can’t be blasé with characters, situations and jokes.’

Released from the creative censorship of a sketch trio, he’s promising ‘all that boring historical detail the others did extremely well to trim out and all those sketches relying on in-depth knowledge of the Battle of Balaclava. I suspect I’ll find my audience over the course of the Festival: men in army jackets with paint on their fingers from decorating their models.’

Despite presenting spoof History is Now videos for Channel 4 online, Ker remains envious of his friend Dan Snow’s television career and maintains his ultimate dream ‘is to do a Michael Palin, for historical rather than geographical exploration. Hopefully, Edinburgh will be a springboard that secures me my own channel interspersed with Blackadder repeats.’

Till then, Festival-goers should check out Nazi Smasher! because ‘I’ll refund the price of their ticket if they do. Wait, don’t write that. No, in the ten years I’ve been working with Dave and Thom, they’ve built up an extreme disdain for their audiences which they hide while they’re onstage. But afterwards they’re real jerks about them. Whereas I’m really cool, I help people move house and that.’

Humphrey Ker is Dymock Watson: Nazi Smasher!, Pleasance Courtyard, Pleasance, 0131 556 6550, 6–29 Aug (not 17), 7.15pm, £10–£12.50. Previews 3–5 Aug, £5.