This is an odd one. I guess this is historical AU, since it's set in an alternate world, but doesn't have any fantasy elements. The world-building is sprinkled throughout, and is largely accomplished by the culture clash of the Sashian embassy staying in the Pueschaian town of Boukos to open trade routes between the two countries, so it comes across more naturally than the info-dumping one might come across in other AU novels.
During a party, the ambassador to Zash goes missing but this isn't noticed until the following morning. His personal secretary Bedar and Bedar's friend Marzana, the captain of the guard, go on a discreet manhunt through Boukous as they try to follow the crumbs of the ambassador's trail after his disappearance. To complicate matters further, the city is preparing for the annual festival to honor their god of debauchery, Psobos, a most un-Sashian ritual. Along the way, they meet a couple of citizens of the town and slowly fall in love over the course of a most unusual night.
This is a charming little tale. I had no idea what I was getting into when I picked it for my Prime loan, and that just added to the charm. Despite the urgency of Bedar and Marzana's mission, this story unfolds at a leisurely pace. There's a spattering of humor, a little action, but mostly it's just two men out of their depths as they navigate an unknown city full of people they don't understand only to discover they're not so different after all.
I thought I had the endgame figured out at one point, but I was only right about part of it. Chereia and Pheres were a great supporting cast and just as fully fleshed out as Marzana and Bedar. The m/f and m/m couples get equal page time to develop and are respectfully handled in the difficulties that faced both pairings. That makes one a little less satisfying than the other, but certainly still an HFN. (To be clear, this isn't Romance and there's no on-page sex.)