Bentley arrives at the Coatzacoalcos River in Central America. The first riverside village they arrive at sends alarm signals upriver. David Lisketter volunteers to make first contact, but takes a pistol with him. Olmec warriors wearing bright coloured feather helmets surround the shore party and David starts shooting in self-defence. His party retreats to the river, just as a fleet of canoes arrives from upstream, trapping Bentley. Olmec warriors and torchbearers swarm over the ship, killing and taking prisoner. Their leader is a huge fat man, a priest-king, wearing a jaguar helmet, and he decides that the pregnant Martha is to be kept, and Pamela, for the colour of her eyes. One of the torchbearers manages to set fire to the wooden Bentley, and the Olmecs evacuate to the village, taking the prisoners and bodies with them. The Olmecs start cooking the dead.
Eagle arrives two days later. The first inkling of the fate of Bentley is when a lookout spots a floating body in American clothes. Marian and her crew launch an ultralight flown by Andy Toffler, the God Help Us.
Marian and her two hundred troops move upriver in forty inflatable motor-boats to the first village. The Olmecs have fled, but alarm drums are sounding. They find the sunken Bentley. GHU confirms the natives have run, and Marian lands her troops. They find where Martha was kept, as she helpfully scratched 'Upstream' and jammed her wedding ring into a timber. They find the remains of Bentley's crew, confirmed by dentistry in the jawbones. Marian gives the troops and the Arnsteins a morale-boosting speech.
Next day the troops are preparing to leave, when David Lisketter staggers out of the jungle. Marian places him under arrest. David confirms the two women were taken upstream the day before, and adds that the natives were grabbing at him and shining lights in his eyes. Ian picks up the clue, confirms with David that his eyes are the same colour as his sister's, then goes into a hut and comes out with a jaguar mask, whose gemstone eyes are the same colour as Lisketter's. Marian orders the village burned. Martha and Pamela were taken upriver with the Olmec leader to the central temple-city. Pamela frets, and Martha surmises that they have fallen into a local myth, which will make sense if they keep their eyes open. As their canoes land and the leader disembarks, GHU appears in the sky behind them.
The first encounter with the Olmec army is on the river; canoes and two big catamarans, loaded with warriors. Toffler in GHU bombs the two catamarans with home-made napalm. Marian and her troops land, and set light to the crops in front of them. GHU reports that a thousand warriors are heading their way. Ian and Marian suppose that the warriors are reacting to what they think the Nantucketters are.
Swindapa watches amazed as Marian 'works a magic': Two hundred armoured troops form up in two lines on a levee, crossbows to each side of spearmen, each man to his place, standing still. Soldiers race out in front with boxes, and scatter the contents. Swindapa has never seen a war band behave so.
The Olmec warriors arrive, and a champion steps out. Marian has him shot at long distance. The Olmecs stop in outrage, then charge, straight over the caltrops scattered previously. They slow down, and Marian opens fire with the crossbows, ordering the troops to concentrate on the men wearing feathers. GHU takes the opportunity to dive-bomb. A handful of warriors reach the novice Americans, and Marian has to charge in with her katana, Swindapa beside her. The rest are driven away in panic by the swords and shields of the crossbowmen.
Martha watches from a prison cell as the warriors return to the city. Warriors call her and Pamela out of the cell and escort them into the temple, where they first watch the sacrifice of the winning team in a sacred game. Martha looks at the panels on the walls as they are escorted on. The myth is explained; A woman is lost in the jungle, mates with a jaguar, and has offspring, part human, part jaguar. The cat returns and eats woman and child, but the child takes over its body and becomes a man-shaped jaguar. Martha puts together what she now knows about the Olmecs and becomes very afraid of the sequence of ceremonies, as she is pregnant and Pamela is part jaguar - at least in the colour of her eyes. The next part of the ceremony sends Pamela into catatonic shock, and they are both thrown back into the cell.
Meanwhile, Marian plans a small-scale cutting-out expedition: Three women, herself, Swindapa, Lt. Hendriksson, and two men, Alonski and Pulakis, all expert with bows or swords. Toffler is aerial scout and distracts the city with fire-bombs, while the five race in, kill the sentries, and race back out with Martha and Pamela, who needs to be carried. More warriors arrive, and in the following melee, Marian gets a spear through the thigh, and faints from blood loss. She wakes on Eagle, where she learns that Alonski was also injured and Pamela was hit by a spear in the ribs, and is not going to live. Her officers have already pulled everyone out and Eagle is sailing back to Nantucket.
Martha meets David on deck, and asks if he has ever had mumps? She explains that she saw some warriors displaying classical early symptoms: David, or one of his gang was a carrier of the disease. She congratulates David on his revenge: the genocide of the Olmecs, not just as the casualties from the disease, but every surviving adult male is likely to be sterile. He has wiped out the Olmecs and all the following Meso-American civilisations he was trying to save.
David howls and throws himself overboard.