and the two cadets raced after
him.

They were in the outskirts of the city now. Great areas were covered
with rolling grass fields where the citizens of Titan spent their
leisure hours playing ball and picnicking, and it was easy for the
cadets to follow the black-suited spaceman. They had to put on their
oxygen masks as the deadly fumes of the methane ammonia atmosphere began
to swirl around them. They were near the outer limits of the atmosphere
screen's effectiveness.

"I think he's going into that building up ahead, Astro," said Tom, his
voice distorted to a low metallic hiss by the miniature amplifier in the
face of the mask.

Astro nodded and they ducked into a gully as Quent Miles turned once
again and glanced down the street.

"Wonder what's in that building?" mused Tom.

"One way to find out," said Astro. "Come on. He's moving again."

The gas began to thicken now, and the two cadets found it difficult to
see more than a few feet ahead as they moved cautiously through the
swirling death around them. After what seemed like an hour, but was
actually hardly more than a few minutes, they found the building Miles
had entered.

"I'd give two weeks' leave for a ray gun now," said Tom.

"Want me to try the door?" asked Astro.

"Go ahead. We can't learn anything standing out here."

Astro put his hand on the circular latch and twisted it slowly. The door
slid back on rollers, exposing a dark interior. The two boys slipped
inside.

"Better close the door, Astro," said Tom. "The ammonia doesn't seem to
be so thick in here."

Astro twisted the latch on the inner side and the heavy door rolled back
into place. They turned slowly and saw a room that was dark except for a
single light gleaming weakly through the haze of the gas. When their
eyes became adjusted to the semidarkness, they moved, searching for
another door in the huge room.

"Are you sure this is the right place?" asked Astro.

"I can't be positive," said Tom. "The stuff outside was too thick--" He
stopped, touched Astro on the arm, and pointed to his left. There was
the sound of a door sliding back and light filtered into the murky room.
Quent Miles stood framed in the doorway, the unmistakable outline of a
paralo-ray gun in his right hand.

"Drop to the floor," hissed Tom.

The two cadets dropped lightly to the floor and lay face down, while
Quent Miles walked toward them fanning the gun around menacingly. Then,
as he was about to step o