He heard a terrified yelp, and the man made a dash past him for the door.

But the door was closed.  Carrington had shut it behind him.  Before the fellow could get it open, Ken was on his feet again, and had flung himself on the signaller.

With a snarl like that of a trapped cat, the man wrenched one arm free.

‘Take that!’ he hissed, and next instant Ken felt the sting of steel grazing his left shoulder.  The sharp pain maddened him, and his grip tightened so fiercely that he heard the breath whistle from his opponent’s lungs.

At the same time he flung all his weight forward, and the other, thrown off his balance, went over backwards and came with a hollow crash against the door.

The two fell to the floor together, and rolled over, fighting like wild cats.

Ken’s adversary was smaller than he, but he seemed amazingly strong and active.  He wriggled like an eel, all the time making frantic efforts to get his right hand free, and use his knife again.

But Ken, aware of his danger, managed to get hold of the fellow’s wrist with his own left hand, and held it in a grip which the other, struggle as he might, could not break.  At the same time, Ken was doing all he knew to get his knee on his enemy’s chest.

It was the darkness that foiled him—­this and the eel-like struggles of his adversary.  At last, in desperation, he let go with his right hand, and drove his fist at the other’s head.  He missed his face, but hit him somewhere, for he heard his skull rap on the floor, while the knife flew out of his hand, and tinkled away across the cement floor.

Ken felt a thrill of triumph as he heaved himself up, and getting his knees on his adversary’s chest, seized him with both hands by the throat.

Before he could tighten his grip came a tremendous shock, and he was flung off the other as if by a giant’s hand.  As he rolled across the floor, followed a crash as though the very heavens were falling.  The whole ship seemed to lift beneath him, at the same time stopping short as though she had hit a cliff.

[Illustration:  ‘Ken flung himself on the signaller.’]

For an instant there was dead silence.  Then from the decks above came shouts and a pounding of feet.  Half stunned, Ken struggled to his feet, and staggered towards the door.  As he did so, he heard the click of the latch, and before he could reach it, it was banged in his face.

Groping in the darkness, he found the handle.  He turned it, but the door would not open.  In a flash the truth blazed upon him.  He was locked in.  The spy had locked the door on the outside.  He was a helpless prisoner in a torpedoed and probably sinking ship.

CHAPTER II

The last of the ‘cardigan Castle’

Ken’s head whirled.  For the moment he was unable to collect his ideas.  He stood, grasping the door handle, listening to the thunder of feet overhead and the shouted orders which came dimly to his ears.