Michael Spencer’s first four victims were shot, stabbed, poisoned and strangled. He wondered then if he could keep things varied. Five years and about 30 victims later, this has not been a problem. Michael continues to kill in a range of ways.

These are fantasy murders, plotted by Michael and left to audiences to solve. Highly Suspect, the Carlisle-based theatre company run by him and his partner Lexie Ward, is about to perform at the Edinburgh Fringe.

Audiences will meet four suspects, played by Michael, Lexie, Seb Coombe and Joe Desborough.

Michael, 33, explains: “The audience is introduced to some incredibly suspicious characters. It’s all fully interactive. The audience are part of it.

“Lo and behold, someone dies horribly - offstage. We’re not paying someone to be in it for five minutes and then lie on the floor! We tell the audience ‘You’re going to have to figure it out.’ They work in teams. We give them crime scene reports.

“The suspects question each other. We tell the audience to listen carefully. The clue is always in a throwaway line. There’s what we call a speed interrogation in which the audience gets to interrogate every suspect. Everyone has to tell the truth, apart from the killer who will lie about anything.

“We love hearing the audience’s theories about who the killer is, especially when they’re wrong. A lot of people stand up when they say who they think did it. They think they’re Columbo! But people don’t want to be ridiculed. We’re laughing with them, not at them.”

Highly Suspect’s philosophy is that their mysteries should be challenging but not impossible to solve.

“There has to be a clear path in which only one person could have done it using one method,” says Lexie, 31.

“When the killer is revealed, you always hear ‘Oh - of course!’ If we have 10 teams you want three to get the right murderer and one to work out every detail. That’s generally what we get.”

In its 21 Edinburgh performances next month Highly Suspect will alternate two of its mysteries: An Austentatious Murder and Gary Jotter and the Magical Murder.

The former has the conceit that a new dating app - Dating Mr Darcy - has been launched using the works of Jane Austen.

“Unfortunately,” says Michael, “its creator was found dead with a broken heart. It’s one of my favourite mysteries. Not only is there a mystery to solve, we help people find love. We create an ‘Austen algorithm’. We get the audience to fill in dating profiles. There are categories for things like caddishness and breeding. ‘Fountainability’: how good a man looks getting out of a fountain and ‘bonnetability’ for how good a woman looks wearing a bonnet.”

Highly Suspect performed An Austentatious Murder on a tour of 15 libraries across the north in 2017. “It was so much fun,” says Lexie. “Backstage we did the dating algorithm sums and worked out who should be paired with whom.”

“Remarkably, it worked!” reports Michael. “People who were already sitting together were found to be each other’s best match.”

Gary Jotter and the Magical Murder is set in Pigblisters School of Magic and Mystery, whose houses include Gryffinwindow and Huffy Puffy Snuffle Truffles.

Any similarity to Harry Potter is entirely deliberate. “There are lots of references,” says Michael. “And for people who don’t get those, there’s a brutal murder to solve!”

As well as partners in crime, Lexie and Michael have been partners in life for 11 years. They met during a Carlisle Musical Society production of Crazy For You.

“My character hated your character,” recalls Lexie. Of the real Michael, she concedes with a smile: “He’s alright.”

Both had been involved in local theatre for several years when, in 2014, Highly Suspect was born.

Michael used to work at Tullie House, which had staged murder mystery events. He asked if he could write and stage one.

“It was unbridled hubris. I’d never even been to one. I thought ‘I’ll be able to do one of those!’ Tullie House had the Crosby Garrett Helmet. So we had the Lesley Garrett Opera Hat. The anonymous donor had been murdered.”

Murder at the Museum was very well received. Lexie says: “Someone had to go home before the end. They rang the museum next day to find out who the killer was.”

Commissions followed, from organisations and individuals. Highly Suspect have performed at everything from corporate events to hen parties. Lexie says: “We go to some incredible places. We’ve just done a 50th birthday in a Scottish castle with full-on turrets and suits of armour. And the restaurant at Durham Cathedral. We had to run it past them to ensure it wasn’t blasphemous.”

“The private ones can be interesting,” says Michael. “The ones who have booked us don’t necessarily tell everyone else what’s going on. We rush in and say ‘There’s been a murder - solve it!’ And people are looking at each other going ‘What?’”

“But by the end they’re desperate to solve the thing,” says Lexie.

They now have more than 30 murder mysteries, all written by Michael, with input from Lexie and the rest of the cast.

Michael says: “At the first rehearsal I say ‘I’ve got parts A and B - help me fill in the gaps.’ Then I go away and write something completely different.”

“My job is to chuck stupid ideas,” says Lexie.

“You just say ‘Blackmail!’ as the motive,” laughs Michael. “I say ‘Why?’ ‘I don’t know!’”

Highly Suspect performed at the Edinburgh Fringe for 11 days last year.

“We weren’t expecting to succeed there,” says Michael. “You hear about shows where the cast outnumber the audience. It was a vanity project for me. I’ve always wanted to do something at the Fringe. We took a punt. The first day there were about 30 people, which was great. And it just grew by word of mouth. For the last few days the place was packed.”

Their productions usually last between two-and-a-half and three hours. In Edinburgh they are condensed into one hour. “There’s nothing lost from it,” says Lexie. “We just talk faster.”

They are performing as part of the Free Fringe. Donations are welcomed. “As I say to the audience, ‘You don’t want four potential murderers cross with you,’” points out Michael.

Beyond the Fringe, other Highly Suspect shows include a western-themed mystery called The Good, The Bad and The Deadly, and The Great British Bloodbath. “It’s a cross between The Great British Bake Off and Breaking Bad,” explains Michael.

Christmas will see the debut of The Gutted Grotto. “The grotto has been giving out wrapped body parts. I’ve got no idea who’s done it yet. And I’ve set myself up to write the most complicated mystery in the world. It’s to do with time travel. Time-travelling characters discover their future selves are dead.”

Lexie is a freelance director who also works with the young actors at Keswick’s Theatre by the Lake.

She has directed several of Michael’s plays, not all of which feature his trademark humour. They include the award-winning thriller We Know Now Snowmen Exist, which premiered at last year’s Carlisle Fringe and was performed in London this year.

“I write stuff, you put it on,” is Michael’s summary of their professional relationship.

“Murder pays the mortgage,” adds Lexie.

“We’ve got so many body parts in the attic,” says Michael. “I do wonder what the neighbours must think. They hear me say ‘I’ve figured out where to put the body!’”

“Highly Suspect by name and by nature,” says Lexie.

* Highly Suspect perform at Bar Bados, Cowgate, Edinburgh, from August 3-24 (except the 15th) at 5.30pm.