Mark of the Plague is physicist-turned-author Kevin Sands’ follow-up to the much-honored The Blackthorn Key, with its teen protagonists facing new dangers and struggling to solve new mysteries in 1665 London. The Black Death has returned to London, spreading disease and fear through town. A mysterious prophet predicts the city’s ultimate doom—until an unknown apothecary arrives with a cure that actually works. Christopher’s Blackthorn shop is chosen to prepare the remedy. But when an assassin threatens the apothecary’s life, Christopher and his faithful friend Tom are back to hunting down the truth, risking their lives to untangle the heart of a dark conspiracy. Who is this stranger who seems to be able to cure the bubonic plague that’s ravaging the city — and what’s in his medicine? As the plot unfolds against a backdrop of the black plague’s devastation, the teens suffer beatings, stabbings, poisonings, and more in their efforts to discover and unmask a murderer. Mark of the Plague delivers swashbuckling adventure, brain-teasing puzzles, several explosions, and assorted gun-brandishing and sword-wielding. Some characters die, and there are brief gory descriptions of injuries and plague symptoms as well as dead bodies. This installment has a strong moral compass, with frequent but non-preachy messages about friendship, loyalty, kindness, using your brain, and doing the right thing even when you’re being strongly pressured to do the wrong one.