“Greek fire,” she said. “It's a combination of resin from local trees. It will burn hotter than napalm. If someone tries to put it out with water, it only makes the fire spread.”

The Trouts looked on with wonder as the flames began to eat through the hulls. They knew the sabotage would help, but once the natives had discovered their scuttled craft, they could race along the well-maintained pathway that bordered the river.

They paired the stronger paddlers with weaker ones. Gamay and Francesca got in one craft. Paul and Tessa took the other. They shoved off into the river and paddled for their lives. After an hour they pulled over to the shore for a drink of water and five minutes of rest, then set off again. The paddles raised blisters on their palms as they pushed the canoes against the river current. Francesca passed around a medicinal ointment from her amazing bag, and it numbed the pain in their hands. They kept on, trying to put as many miles between them and the village as possible before daylight failed.

Darkness came all too soon. Travel on the river became difficult, then impossible. The canoes became tangled in thick grass or ran aground on sandbars. They were quickly exhausting them selves and getting nowhere. They gave up and paddled closer to shore, where they dined on jerky and dried fruit. They tried unsuccessfully to sleep, but the dugouts served poorly as beds, and they were happy to see the gray light of morning.

With bleary eyes and stiff joints they set off again. The sound of drums spurred them on and made them set aside their aches and pains. The ominous drumming seemed to come from everywhere and echoed through the forest.

The canoes glided through the curtain of mist rising off the river. The smokescreen hid them from Chulo eyes, but they had to move slowly to avoid obstacles. As the sun rose it baked the mists off to a translucent haze. With the river ahead once more visible they paddled furiously until the sound of drums faded. They kept moving for another hour, not daring to stop. Before long they began to hear a different sound. Gamay cocked her ear. “Listen,” she said.

From a distance came a low roar, as if a train were speeding through the forest.

Francesca, whose serious expression had not changed since they left the village, ventured a slight smile. “The Hand of God beckons.”

With spirits renewed, they forgot they were tired and hungry and that their buttocks were numb and dug their paddles in once more. The roar grew louder, but it didn't obliterate another sound, a quick whirr as if a river bird had taken flight, followed by a solid tunk.

Paul looked down in disbelief. A three-foot-long arrow was embedded in the side of his canoe. A few inches higher and it would have pierced his rib cage. He looked toward the shore. Flashes of blue-and-white-painted bodies could be seen darting between the trees. The ululating war cry filled the air.