The Investigators are contacted by a small-time chiseler named Kevin Robinson, who wants to sell them a curious stone box. He stole the box from a local mobster named Alfonzo Toletti.
Investigators meet with Professor Lischka, who tells them that the box is antique Inca from Peru. Kevin appears, badly beaten up, delivering a message from Toletti. He wants to meet the investigators for dinner at Hunans Restaurant. After an outward show of pleasantness and guarded negotiations over the box, both sides leave unsatisfied. In an alley outside the restaurant, the investigators are beaten up by Tolettis goons.
The next morning, Al Neville finds Kevin shot through the head on his doorstep.
After staking out Tolettis posh home, the Investigators observe Toletti and his friend Dumont killing the members of a rival mob with guns and magic. Dumont makes cryptic gestures with his hands, which leave the rival mobsters writhing in agony. It becomes clear that Dumont is assisting Toletti with his criminal activities, in exchange for Toletti obtaining rare magical items for Dumont.
Al Neville finds Tolettis girlfriend Lynn shot through the head.
Trailing Dumont in the hope of getting some answers, the Investigators observe him outside a library. Dumont appears to go crazy, wandering around looking at objects as though seeing them all for the first time. Rather than rush up to question Dumont, the Investigators simply observe his behavior from a distance.
The investigators break into Dumont apartment and take some odd metal plates and some figurines. The metal plates bear some kind of strange designs, and what might be writing.
Investigators meet with Professor Lischka again, hoping he can help them understand the plates they found at Dumonts. He gives them a journal article which shows a picture of writing like that on the plates. The article talks about a program called TAMIPS, The Arts Meet Insane People. The writing shown in the article was done, according to the caption, by a mental patient named Theodore Hubbard. He made the writing as part of an art therapy program.
Tracking down the leads from the article, the Investigators meet with Professor Cridlowe. He is Hubbards psychiatrist. He tells them about Hubbard and his condition and where he is kept.
Investigators break into the University Asylum and free Theodore Hubbard: Ex-Yithian. He is quite a handful. Friendly and likable, but prone to fits of panic in which he attempts to flee. They take him to Nevilles home and cajole him with cookies and other inducements, and eventually he explains to them his memories of the hideous nightmare of Yithian captivity. Finally they show him the plates they found at Dumonts, and he is able to translate them. It seems Dumont is planning to summon Shub Niggurath, and needs the various magical items Toletti has been accumulating to pull it off.
With Hubbards help, the Investigators plan their attack. They obtain an obsidian knife, which is the only thing that can harm Dumont in his Yithian state. Going to a local cemetery one night, where they know Dumont plans to perform his ritual, they encounter the mobsters and the Yithian. Hubbard attempts to defeat the Yithian with magic, and dies in the attempt. But he gives the Investigators the opportunity they need to strike with the obsidian knife. The Yithian is dispatched, the real Dumont is restored to his now wounded and bleeding body, and the mobsters scatter.