By day, Vivienne is Guinevere's lady-in-waiting. By night, she's Merlin's secret apprentice, indulging in the new mechanical arts and science of alchemy. It's a preferred distraction from Camelot’s gossipy nobility, roguish knights, and Lancelot’s athletic new squire, Marcus, who will follow in all knights’ footsteps by taking a rather inconvenient vow of chastity.
More than anything, Vivienne longs to escape Camelot for a future that wouldn't include needlework or marriage to a boorish lord or dandy. But when King Arthur's sorceress sister, Morgan le Fay, threatens Camelot, Vivienne must stay to help Merlin build a steam-powered weapon to defeat the dark magic machine Morgan will set upon the castle. Because if Camelot falls, Morgan would be that much closer to finding the elusive Holy Grail. Time is running out and Morgan draws near, and if Vivienne doesn't have Merlin's weapon ready soon, lives would pay the price, including that of Marcus, the only one fast enough to activate it on the battlefield.