In a city that was, relatively recently, stolen by giant bats, a young man wakes up in a holding cell. There's a guard standing watch, though a rather scrawny one.
The guard worries their lip.
Then they seem to make a decision. "Fuck the sergeant's nephew," they decide. "He probably deserved whatever you did to him, and I'm not sending a kid like you to the Spike until you're my age."
They remove a key from their keyring and toss it into the cell. "Anybody asks, you stole that from me. Got it?"
He waits until there's no one around, lets himself out, locks it back up behind him because why not, tosses the key back into the cell just to be annoying, and leaves.
No one stops him - a purposeful stride does wonders.
Outside the door of the jail, the city of Fallen London spreads open before him. Watchmaker's Hill, specifically. There's a pub, and an office building with a sign out front reading DEPARTMENT OF MENACE ERADICATION, and at the top of the hill there's an observatory. Around the hill there are marshes, thick with tall treelike mushrooms.
Inside, there's a bored-looking man playing solitaire at a wooden desk, along with a few grizzled old men with harpoons chattering with each other. Behind the desk are various posters.
RATS: 11 ROSTYGOLD PER 10. WORRYINGLY LARGE RAT: 200 ROSTYGOLD. SORROW-SPIDER LEGS: 15 ROSTYGOLD APIECE. SPIDER-COUNCIL: 1000 ROSTYGOLD. THE VAKE: 1 MILLION ECHOES.
The Apathetic Secretary looks up from his solitaire. "What you here for, eh?"
"...how big does a rat have to get before it's worryingly large?" he wonders, peering at the assortment of posted rewards.
"That's a bounty on a specific rat," the Apathetic Secretary clarifies. "We call it the Worryingly Large Rat because it's pretty concerning that a rat can get that big. The Department want it dead, but the reward they posted isn't enough for the big-time ratkillers to go for it, so it's sitting there until somebody really needs two echoes' worth of rosty."
"Okay, that's pretty big," he acknowledges. "What about the Vake, what the hell's that?"
The monster hunters go quiet.
The Apathetic Secretary looks, for once, engaged. "Nobody quite knows. It's like a bat, they say, but very big, and deadly as anything. It preys on Londoners in general, but its favorite prey is hunters - specifically, Vake-hunters. People come in and say they're after the Vake, and next thing anyone knows they've vanished without a trace."
...he laughs. "Wow, a giant bat that eats people. Why does anyone ever admit they're going after it, then?"
"How does anyone know where to find it if it kills you as soon as you admit to looking?"