Major Characters:

• The Shadow/Lamont Cranston
• Professor Morley: A college professor who takes his students on a field trip.
• The Boss: The head of a criminal operation. Wears a gas mask to hide his face.
• Bronson: The right hand man of The Boss.
• Gaffney: A member of The Boss's operation.

Featured Agents:

• Margo Lane

Minor Characters:

• College Students
• Tour Guide
• Harry (member of the Rescue Squad)
• The cook (who works for The Boss)

Synopsis: (Warning! Contains spoilers!)
Professor Morley takes his class of 17 students on a field trip to view the caverns. As part of the tour, they are taken to see a pit, known as the Temple of Vulcan due to the vapors rising from its depths. At the bottom of the pit lies a pool of water 100ft in diameter, and known to be bottomless. Excited, the students lean over the protective railing to view the area. Suddenly, to everyone's horror, the railing gives way and the students plunge into the pit.

At the airport, the following day, Margo runs into the professor. He is distraught about the loss of his students, and the fact that he must face their parents who are arriving today. An extensive search of the pool at the bottom of the pit has revealed no bodies. Meeting up with Lamont (who was on the same plane as the students' parents), Margo discusses the cavern accident. Lamont's curiosity is peaked and he decides to investigate. They arrive at the caverns and explore the area. Climbing down a rope ladder to the bottom of the pit, they stumble across a delirious student. Lamont calls for a rope to be thrown down to lift the young man to the surface. As the student almost reaches the surface, the rope breaks and he falls to his death. A careful examination of the rope reveals that it was cut. Lamont theorizes that the student was murdered to stop him from exposing the truth behind his missing classmates.

Deeper within the labyrinth of caverns a criminal known as The Boss, who wears a gas mask to hide his face, speaks with his right hand man, Bronson. Bronson was the one who had cut the rope to silence the student. They both meet up with the cook, who works for them. The cook is a little reluctant to stay in the caverns and voices his opinion, but he is threatened by The Boss. As The Boss leaves, the cook is confronted by The Shadow, who asks for his help. He tells him about the lower levels of the caverns, but before he can reveal the truth about The Boss, he is murdered by him. The Boss then turns on The Shadow and threatens him as well. The Shadow declares war on The Boss.

Returning to Margo as Lamont, The Shadow has a theory: perhaps there was a secret doorway into the lower levels of the caverns where the students were taken after the fall. They both search for and find the door, which leads into a labyrinth. Lamont suggests that Margo remains at the door while he explores the labyrinth, that way he can find his way back by the sound of her voice. As he travels down a passage, he finds that it is getting harder to go further; all the while he calls back to Margo. Suddenly, he hears her screams and rushes back. She has been kidnapped.

The Boss and Bronson lock Margo up in a vault. The Shadow comes to her rescue when they leave. As he frees her, they both hear the criminals talk about a mine. The Shadow tells Margo to contact the police.

The truth about the students is now brought to light: they were kidnapped to work the rich ruby mines deep within the caverns, and The Boss will get wealthy off their labor. The students are treated miserably, and are beaten if they don't cooperate. One of The Boss's men, Gaffney, is questioned about the escape of one of the students under his charge. Gaffney angrily replies that he should have let them all go; they have been worked without rest for the last 18 hours. Furious, The Boss grabs a whip and beats Gaffney unconscious.

The Shadow arrives to save the students, but The Boss thinks he's already defeated The Shadow. The mine is filled with a gas that destroys one's willpower, which is why he and Bronson wear gas masks. If they wait long enough, The Shadow will lose his mind-clouding power and become visible. The Shadow counters with a revolver aimed at The Boss. The Boss pulls out a match and threatens to blow up the mine and everyone in it (as the gas is explosive). Even if The Shadow shoots him, the spark from the firing gun will ignite the gas. Quickly, The Shadow, still invisible, swipes the matches from The Boss. Gaffney comes to and kills both The Boss and Bronson.

After their deaths, Gaffney exposes The Boss as none other than Professor Morley. The Shadow tells him that the law will be lenient on him despite his murder of the criminals. The students are saved.