The Shadow Over Walmington is a four-part Call of Cthulhu campaign for seven players, set in the summer of 1940 in a small seaside town in the south of England. The tone should be a balancing act between gritty wartime drama, Lovecraftian horror, and lighthearted comedy. This may sound unlikely, but it works remarkably well.

The idea for this campaign came to me in the spring of 2014, on the way back from a games convention. My travelling companion Laurence and I were passing the time on the long drive by tossing back and forth some ideas for games we'd like to run. I was musing about a game based on the 1942 Ealing Studios film Went the Day Well? (if you haven't seen it, go and watch it now - you can thank me later), but with the characters from Dad's Army. This seemed like a fairly obvious bait and switch; Went the Day Well? is a remarkably bleak film for its time, whereas Dad's Army presents a rather cosy view of the home front in WWII. Mixing the two would provide ample opportunity to confound player expectations.

Laurence looked thoughtful and said "of course everything's better with Cthulhu", and we looked at each other, and that was how it started.

Four hours later, I had the bones of a campaign which was based on three complementary ideas:

• Could I juxtapose the light-hearted comedy of Dad's Army with the far grittier depictions of wartime Britain that were present in films about WWII that were made during or shortly after WWII?
• Could I juxtapose the existential (but fictional) threat posed by Lovecraft's Deep Ones, with the existential (but very real) threat posed by the German armed forces on the other side of the Channel between 1940 and 1944?
• Could I mash-up many of the British books, films and radio programmes created about WWII before (roughly) 1950 into something even vaguely resembling a coherent whole?

The third idea really depends on a relatively small group of British actors who seemed to dominate the British film industry during the quota quickie years. These actors appeared again and again, often in quite similar roles - what if those roles were all the same character? As a consequence (see the portraits below), it's possible to name the actor who 'plays' almost every NPC in the campaign.

In terms of mood, I chose not to go down the weird science or pulp adventure route for a WWII/Cthulhu game - Achtung! Cthulhu already does that very well. Instead, I've aimed for a degree of verisimilitude; the campaign follows an accurate timeline for 1940 (with the addition of Unternehmen Seelowe, which was at least planned for the autumn of 1940).

I went a little overboard on the props, considering this was only for a TTRPG, but I wanted to give my players a sense of immersion by providing them with paper props that were based on 1940 originals wherever possible (the newspaper, for example, contains stories adapted from local Kent and East Sussex newspapers in 1940). Some of the props evolved over the course of the campaign. The ration books started out as empty covers only. I didn't think of making newspapers until the second episode. My initial plans for St Aldhelm's church hall were rather rushed (and were based on vague memories of the TV series), and I made certain obvious errors (principally missing out the stage) that my players have yet to stop ribbing me about.

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The details of the campaign are presented below as a Tiddlywiki, a single-page HTML application that behaves like a full-blown wiki; all of the content is broken into small chunks ("tiddlers") that are linked together. The core information about the campaign (the synopses of the four episodes) is in the Plot tiddler, but there are also tiddlers on every NPC, group, piece of equipment and notable location, as well as copious notes on the sources that I've wilfully mashed together.