It was supposed to be a utopia, a city in the sky where only the elite lived, Ciudad de Cielo is the place, usually abbreviated to CdC and sometimes other names. There people work to advance humanity. Only sometimes the work is a load people have issues with, sometimes the pay isn't enough and sometimes the ordinary folk need a break. So an underworld has developed and part of that underworld is Nikki Freeman, a jaded cop whose alter ego Nikki Fixx is well known in the underworld. CdC is a place with low crime and low murder rates until a murder sends everyone into a tail spin. Alice Blake is sent from earth to help investigate but she has no idea who to trust There are factions and plots and more bodies.
Well that was an interesting twist on the locked room mystery, even if it's a bit frayed around the edges of that trope. Fairly believable overall.