Shoulder to shoulder with Captain Riggs, Don Winslow made for the engine room ladder. In their wake hurried the medical officer and Lieutenant Red Pennington. Mercedes, at Michael Splendor’s insistence, stayed behind in the cabin.

Not one of them believed that the “accidents” reported by the yeoman were at all accidental. With Scorpion spies aboard trouble could be expected from any quarter. Unfortunately, there was no guessing in advance where disaster would break out; or treachery, for that matter. Even as the Gatoon’s afterguard was bending over Ahern’s twisted corpse in the engine room, a shadowy form slipped into the radio shack, abaft the galley. In the faint glow of a shaded bulb, the man’s face was a mere blur. Only his hands showed in dark outline, as they fingered a pair of invisible dials.

Abruptly the fellow sat down, his right hand now concealed beneath the table. A faint, almost inaudible clicking began spelling out in International Morse Code: “SCP—SCP—Acknowledge—SPC—SPC....”

Almost immediately came the reply—a hoarse, murmuring voice from outer space: “Go ahead SC-3 with your report.”

Again the faint ticking filled the tiny room.

“Orders carried out,” it spelled rapidly. “Engines disabled for the next twenty-four hours. CS-3.”

There was silence for a full minute. Then the voice in the radiophone breathed harshly: “The master is pleased. Stand by at midnight for further instructions. That is all.”

Below decks the Gatoon’s medical officer rose, white-faced, from his examination of the Chief Machinist’s Mate.

“This man is dead, though the body is still quite warm!” he stated. “I should say he had been strangled by a noose of thin wire, which became embedded in the flesh. Somebody removed the thing before we got here.”

Lieutenant Allen, catching the captain’s glance of query, shook his head.

“I didn’t touch Ahern, except to turn him over, sir,” he declared. “And there was no one else in the engine room when I arrived. Strange way to kill a man, with a wire noose!”

“The French call it 'La Garrote,’” observed Don Winslow, stooping to pick a peculiar metal object from a dark corner of the deck. “If I’m not mistaken, this thing is it! Looks as if the killer dropped it in his hurry to get away.”

At his words the others turned to stare in fascinated horror. The death instrument was a loop of extremely thin but tough steel wire, threaded through a small metal hand grip. A sharp pull on the latter tightened and locked the strangling noose in the same motion.

“See!” remarked the young Intelligence Officer. “A man could hide this weapon in his closed hand, or slip it into a watch pocket. It’s deadlier than a knife in the back. Look here, Captain! No need to let the crew know just how Ahern was killed, is there? A thing like this could demoralize a ship’s company in no time!”

The grizzled ship’s master met Don’s look in thoughtful silence.

“I understand, Commander,” he said at last. “Knowing what we do, every enlisted man aboard would be suspecting his mates—afraid to turn his back for a second, for fear of feeling his wind shut off! You’re right about keeping it quiet. We’ll put poor Ahern in the bos’n’s locker, and take the key away. Then the engine crew can get busy at that broken steam line.... How long before we can get under way again, Lieutenant?”

Lieutenant Allen shook his head.

“If it were only the steam line, I’d say four or five hours, sir,” he replied dubiously. “But I saw what looked like emery dust near the main shaft bearings. If any of that stuff’s been used we might not make port for a week, if then. All depends on what we find in the next hour, sir!”

“And on how close a watch we keep after that, Captain!” put in Don Winslow. “I’d suggest an armed guard be stationed at every vital part of this ship. Lieutenant Pennington and I will help you keep watch topside, sir.”

Tossing a wink over his shoulder to Red, he turned to the ladder leading on deck.

Sometime after midnight the two young officers stood shoulder to shoulder in the shadow of a port lifeboat, the wind blowing their whispered words out to sea.

“Got your automatic handy, Red?” Don asked casually, resting an arm on the ship’s rail. “We’re part of that guard I mentioned to Captain Riggs, you know. The difference is that we’re not stationed anywhere in particular.”

“I cleaned and loaded my gun before mess gear blew this evening,” young Pennington answered. “But, say! Do you really think there’ll be another attempt to put the Gatoon out of commission?”

“I do,” Don replied, “though mere sabotage wouldn’t be the Scorpion’s real object. He doesn’t go in for small-time stuff. He’d like to sink us all without a trace, and if I didn’t know that we’d destroyed his pirate submarine....”

“But maybe he’s got another we don’t know about!” cut in Red excitedly. “With her engines crippled, this old cutter’d be an easy mark for a torpedo. Or for any armed yacht the Scorpion might have handy. Say, Don, I’ll bet that’s the answer!”

“Keep your shirt on, Red!” Don Winslow laughed softly. “Your theory sounds okay, if you say it fast, but don’t let it scare you off on a wrong tack. I’d stake my commission, on there not being another enemy submarine in these waters; and as for an armed yacht attacking us—well, the Gatoon’s guns outrange any but a destroyer’s. No! There’s some worse danger afloat, and it’s up to us....”

Don’s words trailed off into silence. Stepping deeper into the lifeboat’s shadow, his form was suddenly blotted out.

“Red!” came his low call, above the slosh of waves against the ship’s side.

At once, the stocky lieutenant moved in the direction of Don’s voice. Feeling his way along the lifeboat’s keel, he felt his arm grasped firmly. An instant later an end of light cordage was pressed into his hand.

“That’s the second I’ve located,” Don Winslow whispered in his friend’s ear. “My hand happened to find the first one by accident. What do you make of it?”

“Why—it’s a boat lashing, Don!” muttered Red, wonderingly. “That means the tarpaulin’s loose, and a stiff breeze would lift it.... Huh! You don’t suppose that’s where the killer’s hiding himself—right in this boat above our heads?”

“He may have hidden anything!” Don answered briefly. “Here, let me stand on your shoulders and take a look!”

As Red braced himself, Don went up, catlike, to grip the lifeboat’s gunwhale. Fishing in a pocket, he produced a small flashlight. It’s beam, thrust under the canvas boat cover, lighted up the whole cavelike space beneath.

Red, crouched in the darkness below, felt Don’s weight suddenly leave his shoulders. Glancing up, he saw his friend’s dim form disappearing inside the boat.

Moments passed, with only a faint whisper of movement from inside the covered lifeboat. Red Pennington waited nervously at his post, alert for the slightest sound of approaching footsteps. If the spy had hidden something of value, the fellow might be coming back for it at any time!

Red’s reasoning was better than his hearing, as a matter of fact. When he did hear the faint step behind him, it was too late to turn. Jerking his head to one side, the stocky lieutenant just saved himself.

A numbing blow descended on his shoulder. With a grunt, Red whirled, his fist coming up in a wicked hook which contacted flesh and bone. The unseen assailant’s gasp of pain came a second before Red’s whoop: “I’ve got him, Don! Come—ugh!”

The thug’s elbow jammed into Red’s midriff, and loosened a perfectly good hammer lock. The lieutenant gagged, lost his grip and his footing together, as the enemy tripped him with a jiu-jitsu trick.

At that second, Don Winslow’s lithe form dropped from above.

Only darkness and the snakelike agility of the Scorpion spy prevented his capture then and there. The man leaped over Red’s body, barely avoiding Don’s rush, and jumped for the rail beyond the lifeboat.

Red, scrambling to his feet, lunged for the boat’s forward end. Without warning there came a heavy splash from overside. Don’s shout, “Man overboard!” followed instantly.

“G-great guns, Don!” Red gulped, bringing up against the rail. “I thought he’d knocked you overside! What happened, anyway?”

“He jumped!” clipped Don Winslow as other voices on deck and bridge took up the cry of “Man overboard!” “Listen, Red! You hustle aft and get a place in the first boat that’s lowered. Don’t tell ’em the whole story—only that someone attacked you and jumped overboard when you fought back. Lively now, before anybody sees us together!”

Badly mystified, Red Pennington trotted aft to the group gathering around Number Three lifeboat. He had a hundred questions to ask, starting with: Why was Don staying behind? On the other hand, orders were orders, and questions would keep until Don chose to answer them.