The missile left his hand with terrific speed. All the frantic strength in his arm was behind it. It flew straight to its mark. The Oan carrying the ray gun dropped like a log. Carlson shoved the heaviest boulders off the ledge. He worked furiously, moving from one to the next. They fell like a thunderclap on the rat men below. But Ro had given the signal too late. Three of the Oan were crushed under the barrage. But one moved too swiftly. He passed under the falling stones unharmed and raced toward the fallen Na. Ro drew back his arm. His pounding heart made it difficult to aim. The stone left his hand in a powerful sweep, but went wild. The rat man was less than thirty feet from Na. When he reached her it would be too late. Ro snatched up another stone. He forced himself to be calm as he took deliberate aim. He made the throw smoothly. The stone sped from his hand. It streaked down on the racing Oan and found its mark in the small of his back. The rat man threw up his arms and collapsed a few feet from his goal. Carlson pounded Ro's back jubilantly. The young Martian smiled at the Earthman's enthusiasm. Then, quieting the elation he felt, he grew serious. "Perhaps our friends have not fared so well," he said with his thoughts. "If we find that they have succeeded, we will have real cause to celebrate." Carlson sobered. "If only they have succeeded," he said aloud. "If Charlotte--" Ro couldn't understand the words, but Carlson's feelings were clear. He could understand that the Earthman would be anxious about Charlotte. He placed his hand on Carlson's shoulder in a comradely gesture. "I have a feeling that all is well," he said, wondering how true his thought would prove. * * * * * The two men left the ledge and retraced their steps back to the valley. When they reached the foot of the cliff, Na was standing there waiting for them. Ro took her in his arms. "My stumbling princess," he sighed. "I don't know how you would exist without me." "I would fare very well," she answered, feigning haughtiness. "I only get myself in trouble to let you enjoy being a hero." A thought transmitted by Carlson interrupted their talk. "We must hurry. They may need us." He had retrieved the ray gun the rat man had carried and was fingering the trigger impatiently. "They have only two of these now," he said, "but they will do plenty of damage." They set off in the direction of the sphere. Ro carried a stone in either hand, ready for instant use. Carlson urged them constantly to hurry. But Ro needed no urging. He led them at a fast pace through the forest. In a short while they could see the gleaming sides of the sphere. Ro signaled a halt. He moved on alone, cautiously. His eyes strained ahead for a sign of the enemy, but all was still. Even at the edge of the clearing, he met silence. Then the door to the sphere swung wide. Grimm stepped out, smiling widely. He waved a greeting.