"Here's the first thing you need to know: all the fairy tales are true. The second, and most important thing... once a story starts, it won't stop on its own. There's too much narrative weight behind a moving story, and it wants to happen too badly. It won't stop until somebody stops it." -Seanan McGuire, Indexing
Saint Patrick's Day is normally full of drunken disorderliness; this year, things have gone decidedly weird. There are reports of little men riding dogs, sharp-toothed women swimming in the Detroit River, and drunks running around ranting about pots of gold. The first wave of police response seems to have been absorbed into the crowd of revelers, and it's time to call in the big guns: you.
You are agents of a secret government agency known as the Memetic Incursion Bureau. Fairy tales are real. Your job is to stop narrative intrusions into Reality - and this one will be a doozy. You have badges, guns, and a little bit of Fairy Tale on your side. They have leprechauns, and it's Saint Patrick's Day.
This is loosely based on Indexing by Seanan McGuire. A quick glance at the book's description should tell you all you need to know.