Hand him over to the Constables
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This page contains details about Fallen London Actions.
It isn't what the rats would prefer, but it will make friends in official places, and it may clear away some outstanding suspicions against you.
A faceless thing in a barred wagon
The Constables who come for him wear black. They don't answer questions, except to promise that the Big Rat will be unable to try on any new faces henceforth. They leave behind a small portion of the Big Rat's rostygold for your trouble.