Mystery writer Cornelia Van Gorder has rented a country house called "The Oaks", which not long ago was the scene of some murders committed by a strange and violent criminal known as "The Bat". Meanwhile, the house's owner, bank president John Fleming, has recently embezzled one million dollars in securities and has hidden the proceeds in the house, but is killed before he can retrieve it.

Immediately after the events of The Desolation of Smaug, Bilbo and the dwarves try to defend Erebor's mountain of treasure from others who claim it: the men of the ruined Laketown and the elves of Mirkwood. Meanwhile an army of Orcs led by Azog the Defiler is marching on Erebor, fueled by the rise of the dark lord Sauron. Dwarves, elves and men must unite, and the hope for Middle-Earth falls into Bilbo's hands.