It's autumn 1963, and there's a dark cloud hanging over London. Literally.
The thick, impenetrable smog is the perfect cover for a killer who's stalking the streets, not that traffic warden, Gabriella Farnsworth, knows that when she's approached by three young boys who say they've found a body.
When her boyfriend, Detective Sergeant James Archer, gets assigned the case, he begins to build a picture of the killer's patterns and becomes increasingly nervous every time the smog settles over the city.
Especially when bodies are found once the mist clears.
Gabriella's diverse contacts on the street provide vital information that helps James understand that these murders are not the start of a killing spree; they are a continuation of one that came before.
They'll have to go back to what happened during the Blitz, to another time when London was shrouded in darkness, to find the clues to catch the man who's returned to kill again.
James and Gabriella know that they have to solve the mystery before the next fog descends, because when it does, the killer will strike again.