Siamun finally accepts that Duat is a prison rather than a refuge. He studies the city's three usable exits and finds them too heavily watched to exploit, especially without Ahmose to create distractions. He briefly considers escaping through the Gleaners' tunnels, but the risk of being seen there is too high, so he returns to the only remaining option: the Infernis overflow beneath the river, despite the danger of acid, pressure, and becoming trapped inside the pipes.

To make the escape possible, Siamun solves the problem of protecting his Vitaeria by cutting open his arm and pushing the discs under his skin. He then spends days testing the poison and training himself to remain submerged, first for seconds and then for many minutes, confirming that the hidden Vitaeria let him survive without air. After another week, he admits he has run out of reasons to delay and goes to the underground river to attempt the escape.

At the overflow opening, Siamun is stalled by memories of his sister's death at sea and by the fear that this attempt may end the same way. Netiqret finds him there, says the Nomarch noticed his Vitaeria and has been watching the river, and reveals that Kiya is her daughter and has been for twenty years. Netiqret begs Siamun to stay, claiming she will help him because she now believes he is part of a larger purpose, but Siamun keeps his distance, tells her to meet him there if he returns, and throws himself into the torrent.

The passage through the pipe is excruciating. Acid burns Siamun's skin, eyes, lungs, and stomach, and the force of the current leaves him helpless long enough that he thinks he is dying and fears he has lost the buried Vitaeria. He finally reaches an air pocket and drags himself out into a vast low chamber filled with canals of green and white water.

There Siamun discovers hundreds of supine, twitching people pinned to stone slabs with pipes inserted into wounds in their stomachs. Their collars bear scarab markings, and the pipework carries toxic water in and clear water out. Hiding behind the bodies, Siamun seizes a lone shabby Overseer and compels her to answer him. She explains that the room purifies contaminated water internally through these bodies, that the cleansed water is sent back to Duat's wells, and that the room's entrance was sealed six hundred and thirty-three years ago. She also confirms that the waste canals eventually lead outside the city and were once used to dispose of workers who could no longer perform their tasks.

Horrified that Duat's drinking water depends on this hidden system, Siamun considers killing the captives to end their suffering but rejects the idea because it could raise an alarm or cut off the city's supply. The Overseer resumes her work as ordered, and Siamun follows the green canal to the far wall, where the toxic outflow disappears again into darkness. Knowing exactly how terrible the next passage will be but seeing no alternative, he steps back into the burning water and continues his escape.