Cynical London detective John Verney (Kemp) is on the trail of an ominous masked serial killer cutting a swathe through London under the guise of a 17th century Plague Doctor. As the bodies pile up, John faces scrutiny from his superior (Yip) who dismisses the crimes as gang killings. To make matters worse, journalists, desperate for a news story, are fearmongering with explosive headlines and outlandish social media videos encouraging public unrest.

As the pressure mounts, John falls into a terrifying web of deception and deadly rituals involving secret cults, Jack the Ripper and the Plague Doctors of old. When his son is kidnapped and his colleagues turn against him, John must take the law into his own hands, risking everything to uncover the ancient evil lurking within the tunnels beneath the city.