July 1922. The investigators have travelled to Vienna, Austria to attend an auction of occult items at the House of Ausperg.
After a champagne dinner with the other clients at the Ausperghaus, the auction begins. At first, nothing seems amiss. But as the seventh lot is brought into the auction room, a terrible scream is heard from the adjoining room. Rushing to find the source of the commotion, the investigators discover that one of the staff has been horribly murdered—the body torn to pieces and thrown around the room. And lot 9—the Riveted Brass Head—is missing.
A trail of blood leads to the vault, where a tunnel has been excavated from the sewers nearby. But there is no longer any sign of the perpetrators or the Head. A strange sickly-sweet smell lingers in the vault.
The police arrive in due course and the investigators overhear some of them discussing the similarities with a series of grave robbings which have occurred recently in Vienna. The old graveyard known as Das Tor (The Gate) is mentioned.
Dr. White obtained some tissue samples from the scene of the crime, and analysing them in the university lab, discovers that the tissue is human-like, but with some disturbing chemical differences. It is unusually resilient and there is an absence of pigmentation. The blood group is unknown.
Over the course of the following days, suspicion falls on one of the guests at the auction, Herr Klaus Hunderprest. He is an Austrian about 60 years old, with a repellent cast to his face. His face is very long and his head slightly pointed. His skin is oily and coarse. Dr. White recalled that during dinner, he blinked very rarely, only once every couple of minutes.
On Saturday 15 July 1922, following the Das Tor lead, the investigators stake out the Das Tor cemetary. A strange figure cloaked in black walks though the cemetary after midnight. Following him, they are led to a dilapidated three-story building on Vohlstrasse. Eventually they learn that the figure is Hunderprest, who rents an apartment in the basement.
Speculating that there are underground tunnels leading from this basement apartment, the investigators crawl through nearby sewer pipes and do indeed discover a network of narrow burrows connecting to the sewer pipes. But they get lost in the labyrinthine warren, eventually emerging 12 hours later in a graveyard far away.
An ill-fated attempt to force the door of the apartment was cut short when a neighbour alerted the police. Brother Bug lithely jumped over the wall into the adjacent yard, Sister Cassia crawled into hiding behind some rubbish bins, but Dewar and Elaine Gibbson struggled to get over the wall and were seized by the police officers. Dr. White threw a molotov cocktail down the basement stairs, then dashed through the first floor door, pushing the surprised manager, Herr Adolf Liebermann, to one side. Dewar and Gibbson were hauled off to the police station.
Sister Cassia approached Herr Liebermann, and with the help of a generous bribe, persuaded him not to press charges. In the end, no-one was hurt and the damage from the molotov cocktail was limited. Dewar and Gibbson were released with a police caution and told to stay out of trouble.
A second attempt to penetrate the burrows is better equipped, with twine, flashlights and a compass. The engineer Dewar leads the way, eventually discovering a shaft leading upwards. Standing up in the shaft, he hears the sound of shifting bodies and a grinding, gnawing sound from above. He points his flashlight upwards, attracting the attention of whatever monstrosity is making that ungodly noise. With superhuman suddenness, a hideous dog-like head appears over the top of the shaft and a human-like creature with sharp teeth and vicious talon-like nails crawls vertically down the shaft, head-first. Before Dewar can react, it slashes at him with one of its clawed hands, severing the artery in his neck. Dewar points his signal flare up the shaft and ignites it, driving the creature back up the shaft into the darkness above. But for the gallant engineer it is too late. With his lifeblood gushing from his neck, he collapses onto the mud of the tunnel floor.
Fleeing for their lives, the other investigators crawl as fast as they can through the muddy tunnels to escape the horror. No-one remembers clearly how or where they exited the tunnels, but eventually they find their way back to their hotel, filthy and despondent at the loss of their friend.
Their brief spell in a jail cell had one good outcome, putting Elaine Gibbson in contact with Vienna’s criminal underworld, allowing the investigators to recruit the services of Jakob Moriarty, a burglar. Returning to Hunderprest’s apartment at night, Moriarty swiftly picks the lock allowing them to enter.
Within, they find Hunderprest in his library. Pointing his revolver at him, Moriarty demands that he raise his hands. Moriarty does so, but begins to utter words in a strange, unfamiliar language. Elaine Gibbson dashes forwards, trying to put her hat over his mouth, but Hunderprest grabs her by the wrist. He pulls out a small jackknife and holds it to her neck, warning the others to stay back. Dr. White rushes forwards, taking a swing at Hunderprest’s legs with his crowbar. Hunderprest crumples, at the same time slashing at Elaine’s neck. She falls to the ground in a pool of blood. Moriarty crowns Hunderprest with his own crowbar, killing him with a single blow to the head. Brother Bug staunches the bleeding from Elaine’s neck, but she needs urgent medical treatment. He carries her out into the street in search of a cab to take her to a hospital, leaving White and Moriarty to search the rest of the apartment.
The familiar sickly-sweet smell emanates from a walk-in closet in the bedroom, prompting Dr. White to search for a trapdoor. Indeed there is one, with a shaft leading downwards and an electric lightswitch near the top. Dr. White climbs down the ladder and is greeted by a scene of horror below. The walls and floor of this room are lined with marble stolen from tombs and mausoleums. Tapestries made of grave shrouds decorate the walls, interwoven with the filthy bones of their previous owners. On a table in the centre of the room stands the Brazen Head. Dr. White shrieks and is overcome with a wave of paranoia. He flees back up the ladder, notwithstanding Moriarty’s attempts to stop him.
Now it is down to Moriarty to discover the horrors of the final chambers. A side room contains a marble slab with the worm-eaten body of a woman lying atop it—Hunderprest’s wife, who he had hoped to bring back to life by necromancy with the aid of the Brass Head.
Searching beyond the door on the other side of the sepulchre, Moriarty saw the final horror—four of the dog-headed humanoids, crouching in a circle, tearing the flesh from human bones with their teeth. Dewar’s dead eyes stare from the ruined remains of his face, while one of the ghouls gnaws on his skull.
Determining to make an end of them, Moriarty tosses Dr. White’s stick of dynamite towards the horrors, dashing back through the door. An explosion shakes the cavern, bringing down the ceiling of the clay tunnels and burying the monstrosities.
Hunderprest’s schemes and his grave-robbing are at an end, and the investigators decide it is time to leave Vienna, as quickly as possible, with the terrible knowledge that monstrous ghouls lurk in tunnels under the city, feeding on its dead.