 While resting on the bank of a swift flowing stream in an uncharted section of the south-central African jungle, Tarzan and Darno, Major Burton Ashley, Jeanette Burton, Dr. Wong Tai and Terrence O'Rourke are captured by a band of half-human giants. They escape and take to the river in a crude dugout. Carried around a bend, a precipitous wall of rock towering a hundred feet above the stream confronts them. Coming from cave-like openings with which the cliff is honeycombed are scores of the yellow savages. Yelling fiendishly, the half-human monsters scramble down the face of the precipice to congregate on the high banks of the river. Rushed forward in the fearful grip of the current, menaced on all sides by the yellow-skinned monsters, the dugout speeds straight and swift as an arrow toward a narrow black opening in the base of the wall. I do. We're going to hit that hole at tremendous speed. Watch out for rocks from overhead. They're going to drop them down on us. These are the rifles. Beat, O'Rourke, Ashley, Wong, Fire. Look out. Here comes a rock as big as a hole. No, if that had hit the... They're going to rush us. Just look out for those stairs. Use your rifles, men. All of you, duck your heads. Here comes the tunnel. Down, down in the bottom of the boat. Hold tight, Jeanette. And if we don't get out, goodbye, my dear. Jeanette's reply is drowned in the roar as the craft shoots into black and utter darkness. The rushing of the waters echoes and reverberates from the rocky walls with a noise like thunder. Onward they speed through the Stygian darkness for what seems an eternity. Pouched in the bottom of the dugout, carried at the will of the flood, fearful that each moment will plunge them over the brink of a cataract or grind them to bits on jutting rocks. Through the bowels of the mountain they rush. And that hole, it looks a long way from us. It looks like a pinpoint. Well, we must be traveling at terrific speed. See how that opening is growing inside. I can see smooth water beyond. Oh, thank heavens. We can only reach solid ground again. Hey, there's something we're shining when we came in here. Look at that reflection on the water. Please, not sunlight. The moon, O'Rourke. We've been in this tunnel a long time. Ta-da! Have you any idea where we are? No more than you have, Dono. That opening is pretty low. We've got to get down on the bottom of the boat again. We're going through in a second. Clear, sweet hair. That sky. I was afraid I'd never see it again. But where's the jungle? Nothing but tall cliffs on both sides of us. No place to land. As the boat shoots out of the roaring cavern, its occupants gaze around in wonder. At their backs is the cliff with the narrow opening through which they've just come. They pour them through a narrow gorge, a swift flowing river. On every side rise tall cliffs, tower-like pinnacles, fluted in ornate columns, slender pillars, bare titanic boulders, and great slabs balanced on their summits. Straight from the riverside, the fantastic rock formations rise, many of them shear for half a thousand feet. Nature's minarets inspire, visible as though viewed through a sheet of silver gauze under the soft, diffused radiance of a tropical moon, round and bright as a new minted coin. Perfectly wonderful. Oh, after rushing through that awful black hole, expecting to be smashed to bits every minute. And that's the seat of the yellow savages from whom I hope we have definitely escaped. I've been thinking about that. Tarzan, do you think those fellas are following us? I doubt it. They would have been out of that hole by now if they were behind us. Then it should be safe to land now. Hey, Lieutenant, boy, land. I mean favorably sticking to this wood skin as long as we can. We can land any time. Oh, and those shear cliffs, Terry? Well, I haven't seen a spot wide enough to stand on yet. But if I am not mistaken, the river is gradually widening. And down there, see, ahead of us is not that a narrow strip of bait, eh? I agree with Terry, my friends. Stay with the boat as long as we can. Perhaps we may yet reach the Congo. Sure, and that's my idea exactly. I'm not in favor of taking chances with them yellow here than again. Just because they're not right behind us is no sign we won't have trouble with them yet. Come to think of it, the biennial, it is rather strange they have not followed us. I wonder why. They certainly must know of that underground river. Have you any explanation for it, Tarzan? No, Miss Burton, but I'm sure they haven't. It may be that this tunnel is taboo, no? That is possible. Strange place this. I doubt if any man has ever been in this canyon before, at least by the root weekend. And for all we know, the exit may be by tunnel also. It is a weird place right enough, Ashley. Unlike anything I have ever seen in Africa, the map shows a cluster of mountains somewhere in this region. But I never dreamed there were anything like this. No, the two toward yellow skin savages lived in here, eh? Yeah, true enough or not. But those fiendish men, I suppose they are men. Double me. I have been thinking about them ever since they saw the one Tarzan killed. They bothered me a lot more for a lever after us than when they had us and they do know. I don't care if I never see any of them again. My settlement's exactly that. Eh, you do not understand what I mean either of you. Who and what they are is what troubles me. I think I understand, Wong. I agree with you. They certainly are not of recent origin. As a matter of fact, I'm firmly convinced that these creatures are survivors of the cavemen. Vous vous pensez, the cavemen, mon majeur? Yes. You saw the face of that cliff we passed under, on a comb with caverns. Those savages live in it. Not only that. Did you examine them closely while they had us back there on the river bank? But majeur, the cavemen, the Neanderthalmen, for example, he was a much smaller creature. Lieutenant, but are we positive of that? There were undoubtedly fads of giants such as these even then. Of course. These creatures are more like huge, hairless apes than men. Didn't you see the flat low craniums, the long arms, the short legs? Wong, there's a marvelous field for investigation here. And for the sake of ethnology, I think we should... Well, you ethnologists are welcome to investigate all you want to. But you'd better wear armor and carry machine guns when you begin your investigation, I think. That may not be such a bad idea, Derek. Well, I have a much better idea, Uncle Jim. Stay as far away from them as possible. Ethnology won't suffer greatly, you know, by not knowing about those things. Mi-vo-la-ta-za! Look there. We are coming out into open country and jungles. The cliffs, they are dropping behind us. Yes. Listen. Numa. Sable. Sheeter. A country filled with unspeakable harm. Death behind each bush. Oh, it's terrible. But were you not warned of those horrors, my dear? I believe the major and O'Rourke, as well as I, endeavored to convince you that the jungle is not the place for a young slight woman. You did for three months. And it was only when I threatened to follow you if you left me behind in Nairobi that you agreed to my coming. It would have been better, Miss Burton, had you listened to your friends and remained behind. Uncle Jim is all I have left Tarzan. I couldn't let him go off into the jungle at long. Listen. Sounds like a waterfall somewhere ahead. Yes. We must land. You noticed how the current has become swifter? In the future we get a show of the better. We can inspect that fall in the morning. Perhaps carry the dead out to the lower water, eh? If we are going to Tarzan, we had better do so quickly. It seems to me that we are approaching that fall or whatever it may be, very fast. Right. Head for that open space on the right bank. Careful of that point, jutting out into the river. If we hit that... We'll have to go around it. Land on the other side. Use your paddle. Keep them deep, men. The current's getting stronger. Paddle, everybody. Once we're in that backwash, we're safe. Paddling doesn't seem too... too much good. We're passing the backwash, Tarzan. She's right. The current's taking us out towards the middle. Very gary. I don't think we'll make it. Paddle, O'Rourke. Ashley. Dono. We are doing the best we can, Monabi. By the due. There goes my paddle. It's snapped off just above the blade. Are there any more in these dugouts? Oh, here, here. Take mine. We'll never make it out of that. Don't give up. Use those paddles. No, Pa, possible. It is useless, Tarzan. We are so near the edge now that nothing but the miracle can save us. Look, Dono. Those limbs. Frances out over the water. When we pass under them, I'm going to jump. Watch for me up ahead. With the speed of an arrow, the long, ungainly dugout races toward the overhanging branches festooned with creepers and vines. As the canoe shoots beneath them, Tarzan leaps high into the air. The steely fingers of one hand unerringly find and grasp the loop of the thick vine. The next instant, with the speed and agility of Manu, the monkey, the eight man is swarming up one strand of the creeper toward the thick foliage above. The dugout speedily leaves Tarzan behind. Nearer and nearer to the cataract to help this group is rushed for the awful current. Ever louder becomes the roar of water, as tons upon tons of it strike the rocks and boulders far below. In the apparently doomed canoe, five hopeful pair of eyes are glued to the eight man as he reaches up to swing himself onto the branch. Suddenly a cry of horror from the occupants of the dugout. Tarzan, about to grasp the limb, hurtles downward toward the rushing stream clinging to one end of the broken vine.