The Werewolf by Night fights Belaric Marcosa -- but how do you kill someone who's already dead?
In the haunted Marcosa House, Jack Russell kneels beside Topaz's dead body, while Belaric Marcosa stands gloating on the landing above, and Buck Cowan hangs from a noose between them. Suddenly, Buck begins to beg Jack for help. Furious with the mental torture he and his friends are suffering, Jack climbs toward Marcosa who vanishes when Jack gets close. Buck's rope comes loose and he falls, breaking his leg. Jack again becomes the Werewolf (despite the lack of a full moon), but retains his ability to think and speak. Buck insists that he is really Buck, explaining he came out of his coma, and learning of his friends' peril, made his way to the mansion. Jack gives him the benefit of the doubt for now, then Topaz's body revives, albeit in a demonic state. Defending himself, the Werewolf slashes her throat and she again falls dead.
Jack and Buck enter the dining room where they are startled and relieved to find Topaz alive and well, focusing her energies on the skull they'd found. As they bring each other up to speed, Buck slumps and Jack carries him to a bedroom to rest. When they open the door, Elaine Marston leaps out and stabs Topaz, then attacks Jack. He punches her and she falls, out cold. Jack rushes to Topaz, but she's merely sustained a minor shoulder wound. Jack's sister Lissa joins the group, and they revive Elaine and treat Buck's and Topaz's injuries.
All except Buck return to the dining room where they witness a struggle between Marcosa and a being who had claimed to be the spirit of the Werewolf. The spirit hurls a mystic fireball at their feet and the floor drops out beneath them. They and the skull fall to the lowest level where they had previously found a headless skeleton chained to the wall. Guessing the skull belongs to the skeleton, Topaz reattaches it, and the spirit rematerializes, but now he is dressed in 19th Century garb. He admits he lied about being the werewolf spirit and introduces himself as Gideon Blaine, a magician whose wife was killed by Marcosa. Seeking vengeance, he too became a victim of Marcosa. He indicates another wall behind which lie the remains of more victims, and Jack smashes a hole through it so the group can enter. They see several corpses from which spirits rise. They offer to give Buck the strength he needs to recover, if Jack and his friends help release them from Marcosa's influence.
They agree and Blaine leads them to a locked chamber where Marcosa's remains lie. However, when they break the lock, giant tentacles whip out to attack them, and the spirits vanish in flames. Jack, still in his lupine form, severs the tentacles and reseals the door.
They then hear police sirens and rush to the entrance, expecting rescue, but are shocked to see the officers are all zombie-like creatures. Behind them Buck appears, and reveals himself as Belaric Marcosa.