What is a short summary of the story, Prince Hat Underground, from the collection, White Cat, Black Dog: Stories?

Prince Hat Underground follows Gary and Prince Hat, a middle-aged couple in an unconventional relationship. When they encounter Agnes, Prince Hat's old friend and former fiancée, Gary becomes concerned about their bond. Weeks pass without contact from Prince Hat, prompting Gary to travel to Reykjavíc in search of him. Guided by a snake named Brennivín, Gary embarks on a journey through dark passages and encounters peculiar creatures, including Sræmundr the Learned. He eventually finds himself in a strange suburban landscape, discovering that Agnes is the Queen of Hell. Agnes offers Gary a deal to spend nights with Prince Hat, but Gary's attempts to wake him are hindered. Eventually, Prince Hat wakes up, revealing a bet between him and Agnes. They leave Agnes behind and return home, but the story hints at a future where Gary will be left alone with debts to pay.