Going to Fourside museum, you'll discover that Mr. Spoon will only show you his discovery if you get an autograph from Venus, who is the latest act at the Topolla Theater.
Give Mr. Spoon Venus's Autograph
Go to Topolla, get a ticket, and watch the show. Then head backstage to get Venus's autograph.
Give Mr. Spoon the autograph. He'll step aside and let you go into the back room where his discovery awaits. It's... a ladder down into the sewer. Gross. Climb on down.
Go through the Sewer and Fight the Plague Rat of Doom
The quickest way to clear this dungeon is to get into the nasty water and slog toward the right. You walk more slowly in the water, so be prepared for this to take a while.
At the end, you find a shiny spot that is the Plague Rat of Doom. Find out how to defeat the Plague Rat of Doom.
Once you defeat it, climb up to get to the next Your Sanctuary. There is also a treasure chest up there.
Once you go back into the sewers, be sure to look all around for treasures. There are plenty here in trash cans. Once you're done, go back up to the museum.