Castle and Beckett investigate the murder of dockworker Donny Hayes. The victim was found floating in the East River with a nasty head wound. A member of Gamblers Anonymous, Donny's bank account had gone dry. He recently cut a $150,000 check to Billy Pitt, an ex-con who claims Donny bought his downtown bar, The Old Haunt. He outbid Brian Elliott, a bartender at the place.
Castle knows The Old Haunt well. He wrote most of his first novel in a booth there. That was before he had Beckett as his inspiration. The place is full of history. It became a bar during Prohibition as a speakeasy. Castle says, "I swear you can still feel the vibration of every notorious episode of glamour and debauchery in its walls." Sounds like a fun place!
Beckett lets down her hair and pops a few buttons on her blouse in an attempt to sweet-talk Brian into showing her the kind of liquor bottle that may have been used as the murder weapon. Her charms work well even after a slightly jealous Castle tries to make it look like they're a couple. It's an undercover thing.
Brian leads Castle and Beckett to Donny's office, which is found in the hidden basement of The Old Haunt. There are buckshot holes in the wall that match wounds Lanie found on the victim. A few weeks earlier, Donny took a baseball bat to the truck of a guy who was getting rough with one of his workers. The customer is known as Pickup Pete because he drives a pickup truck which just happens to have a gun rack. Donny threw Pete out of his bar again the night he was killed.
Castle ponders the idea of buying a bar of his own. He'll call it The Castle. There will be a drawbridge at the entrance. Castle-blanca is another possible name. But before Castle can start making out the drink list, Ryan and Esposito bring in Pickup Pete, who claims Donny gave him a thousand bucks for the damage he did to his truck. Apparently, Donny had a wad of big bills. So, where was all this cash coming from?
Donny was doing business with a high-class auction house. Associate Director Steven Heisler says he obtained a valuable bottle of whiskey that belonged to a former New York mayor who opposed Prohibition. The scotch was left to Donny by his grandfather, Leo. It sold for $26,000. The red glass bottle matches the shards of glass Lanie found in the victim's head wound.
The bottle that sold at auction is recovered intact. This leads Castle to believe there's more than one out there. He also figures out how Donny was shot at The Old Haunt. There's a hidden wall in the basement office. An old shotgun was rigged as a booby trap behind another hidden door that leads to sewer tunnels. Castle and Beckett find the secret room holding lots of bottles of the old mayor's valuable scotch. One of them has been broken. Looks like they found their murder weapon!
Donny was killed in the secret room and his body was ditched in a sewage disposal area. As Castle and Beckett search for more clues, they are startled by an intruder. They give chase but lose the suspect who seems to have disappeared into a dead end brick wall. Castle says there are plenty of old sewer lines that were bricked up when the new ones were established. The killer likely obtained access using one of those old passageways.
Castle and Beckett realize someone has been checking out sewer maps in the city's archives. Any guesses as to who? Going once. Going twice. Sold! To whoever picks the guy who runs the auction house where Donny sold his booze. Steven Heilser is in the middle of an auction when he sees Castle hold up a bidding paddle in the back of the room. He's not alone. Beckett and her team are there, but they aren't holding paddles. No, they're holding badges. This auction, and this case, is officially closed.
In a side story, Alexis has trouble bonding with her long, lost friend Gracie, who has gone goth over the years. But the two eventually embrace the changes they've gone through since they were 10. Speaking of changes, Castle is making one of his own. He bought The Old Haunt! He invites the gang over for a drink. They all exit the station while singing "Piano Man." It's a little off-key, but it's better than drinkin' alone.