What is the main conflict in Neverwhere by Neil Gaiman?

Londoner Richard Mayhew stops to help an injured girl and becomes lost in a shadowy parallel world, London Below. The girl, Door Portico, is on a mission to find out who murdered her family, and reluctantly Richard goes with Door on a quest to solve the mystery. Through many trials, they finally discover that an angel, Islington, is responsible, and they manage to send Islington off into a faraway realm. Door sends Richard back to the normal London, but soon he realizes he must return to London Below.