The Keep starts in an empty citadel with nickel-laden walls holding an entity called Radu Molasar. Molasar. When the German Army headed by Capt. Klaus Woermann takes over the place, the soldiers accidentally release the being when they pick off a coin from one of the walls. As days pass by, Molasar starts killing off the soldiers one by one. The toll in the death of the soldiers leads to the arrival of an Einsatzkommandos detachment group commanded by SD Sturmbannfuhrer Eric Kaempfer to address the issue, punishing a number of villagers along the way.
Persuaded by a local priest, the German soldiers seek the help of the Jewish historian, Prof. Theodore Cuza. The professor examines the writings on the walls of the citadel and decodes the message encrypted on it. Meanwhile, when the professor’s daughter is assaulted by some members of the detachment group, Molasar comes to her aid. Molasar also heals the professor’s skin disease. The professor realizes that he now owes something to the entity while Molasar slowly forms a body. Then out of the blue, s stranger named Glaeken comes and faces off with Molasar. Molasar loses his strength and returns within the walls of the citadel while Glaeken takes a permanent place on the spot desecrated previously by the German soldiers.