Just beyond the Gilded Age, in the mist-covered streets of New York, the deadly Spanish influenza ripples through the city. But with so many victims in her close circle, young socialite Allene questions if the flu is really to blame. All appear to have been poisoned—and every death was accompanied by a mysterious note.

Desperate for answers and dreading her own engagement to a wealthy gentleman, Allene returns to her passion for scientific discovery and recruits her long-lost friends, Jasper and Birdie, for help. The investigation brings her closer to Jasper, an apprentice medical examiner at Bellevue Hospital who still holds her heart, and offers the delicate Birdie a last-ditch chance to find a safe haven before her fragile health fails.

As more of their friends and family die, alliances shift, lives become entangled, and the three begin to suspect everyone—even each other. As they race to find the culprit, Allene, Birdie, and Jasper must once again trust each other, before one of them becomes the next victim.