I feel like I’ve betrayed everyone in Ely.

My next book is not going to be about Philip Dryden – the city’s very own sleuth-reporter, despite the fact that all the readers I meet on the street want him to return. He will. But not quite yet.

Next week, a new series starts, and it’s set just down the road in Cambridge.

My new hero is Detective Inspector Eden Brooke, and despite being very different from Dryden I think they’d be friends. They’d certainly make a great team.

The problem with Cambridge is avoiding the cliché – a parade of punts, golden colleges, mortar boards, irascible porters, students feasting on stuffed swans.

So I’ve set the book in the Second World War. The city – a town then – lay at the centre of the war effort. Thousands of evacuees crowded in, along with civil servants up from Whitehall. Shelters were dug on Parker’s Piece.

Scientists worked on secret projects, local factories switched to producing vital electronics. The city was bombed; more than 1,200 homes were damaged in raids, and thirty killed.

Brooke is no ordinary copper. A forty-year-old veteran of the Great War, in which he fought with Lawrence of Arabia in the desert, he was captured, and tortured. He suffers from chronic insomnia; today we’d say it was post-traumatic stress syndrome. His injuries rule out a return to being a student, and so he joins the ‘Borough’ – Cambridge’s miniature police force. At night he strolls the city of his childhood, its alleys and courts, enjoying the company of fellow ‘nighthawks’ – condemned like him to work after sunset.

Part of Brooke’s rehabilitation demands a regular swim, which he takes in the Cam, a ‘wild swimmer’ before his time.

One night, during the first great Black Out of the war, he spies a platoon of soldiers on the towpath, carrying spades, dragging empty carts from the riverside meadows. What have they left buried in the pits now shrouded in darkness?

What can explain the terrible injuries sustained by a man found dead the next day on the riverbank? The answer is not the solution to a single murder, but a warning of the shape of things to come in a century of war.

The Great Darkness launches on March 1. Tickets from Toppings Bookshop.