This story is a sequel to The Pony Dreadfuls

Years after the events of the second changeling invasion of Canterlot, under the reign of Princess Twilight Sparkle, an earth pony detective stumbles into the web of secrets and lies surrounding surroundingEquestria's last, worst defense against the unknown...

Part of the Silververse and takes place alongside the events in Putting on a Silver Robe and Wizard Hat. However it can be read entirely as a stand-alone story.