Finally he picked a large plastine container filled with a white, crystalline powder. Then he selected a couple of bottles filled with a clear, faintly yellow liquid, and took a hypodermic gun from the rack. He relocked the cabinet. Suddenly a knock sounded. He stiffened, sucked in his breath, and turned to face the door. "Who's there?" he asked cautiously, trying to counterfeit Stevelman's voice. "Harrenburg," said a rumbling voice. "I'm on guard duty. Heard some noise coming from in there a while back, and thought I'd look in. Everything all right, Dr. Stevelman? I mean--" "Everything's fine, Harrenburg," Wayne said, imitating the medic's thin, dry voice. "We're running some tests on Captain Wayne. They're pretty complicated affairs, and I'd appreciate it if you didn't interrupt again." "Sure, sir," the guard said. "Just a routine check, sir. Colonel Petersen's orders. Sorry if I've caused any trouble, sir." "That's all right," Wayne said. "Just go away and let us continue, will you?" There was the sound of the guard's footsteps retreating down the corridor. Wayne counted to ten and turned back to the things he had taken from the cabinet. The bottles of liquid and the hypo gun went into his belt pouch. He tucked the big bottle of white powder under his left arm and cautiously unbolted and opened the door. There was no sign of anyone in the corridor. _Good_, he thought. It was a lucky thing Harrenburg had blundered along just then, and not two minutes later. He stepped outside the Medic Section and locked the door behind him with the key he'd taken from Stevelman. After turning the needle gun back to low power again in order to keep from killing anyone, he started on tiptoe toward the stairway that led into the bowels of the ship. After about ten paces, he saw a shadow on the stairway, and cowered in a dark recess while two crewmen passed, talking volubly. Once they were gone, he came out and continued on his way. It took quite a while to get where he was going, since it involved hiding and ducking two or three more times along the way, but he finally reached the big compartment where the water repurifiers were. He climbed up the ladder to the top of the reserve tank, opened the hatch, and emptied the contents of the jar into the ship's water supply. "That ought to do it," he said to himself. Smiling, he carefully smashed the jar and dropped the fragments down the waste chute. He surveyed his handiwork for a moment, then turned and headed back. He hadn't been seen going down, and he didn't want to be seen going out. If anyone even suspected that he had tampered with the water supply, all they would have to do would be to run the water through the purifiers. That would undo everything Wayne had been carefully preparing. * * * * * He made his way safely back up to the main deck and headed through the quiet ship toward the airlock. He wasn't so lucky this time; a guard saw him.