In the small town of Exham, not far from the city of Arkham, a mysterious epidemic has broken out. Rats are everywhere, and people infected by them are taken from madness. It's up to a biology researcher to find out what's going on, teaming up with a resolute veteran of the First World War. But the two have no idea of the chain of events they are going to discover, going up to a family tragedy and down to the sewers, where a strange creature is lurking in the shadows.