Fourteen-year-old Maggie Ogden thinks she knows everything there is to know about Grandfather Clement Clarke Moore and his legendary poem, Twas the Night Before Christmas. All this changes Christmas Eve 1854 when Chelsea Manor receives an unexpected visitor - Henry Livingston; a young man searching for evidence to provide his grandfather, Major Henry, is the true author of the famous holiday poem. The night takes a strange turn when Maggie and Henry come across another intruder in Chelsea Manor. After following a peculiar boy down a secret opening within the fireplace and through an underground tunnel full of mechanical sleighs, Maggie and Henry discover Poppel, a hidden village under New York City. Faced with the deadly challenges presented by the legend of The Three Sisters, Maggie and Henry are forced to play by Poppel's rules and those who run the cursed village. And they quickly learn that not only is leaving Poppel harder than arriving, but there are even more dangerous lies and mysterious truths to Clement Clarke Moore, Christmas, and the menacing creature, Krampus, than anyone could have known.