Qart is a bright, filthy mouthed, street boy who has run away from Carthage to make a new life on the Great Sea. He lives in a dockside shanty in Ibossim in the West and gambles with spinning tops to make ends meet. His existence is so precarious that when he meets Dubb of Miletus, a kind if awe inspiring navigator, he decides to join forces with him, whatever the cost.
Dubb has his own worries and neither time enough, nor inclination, to become the friend of a troublesome runaway. His daunting task is to navigate a vessel, The Delphis, through the infamous waterway known as the Pillars Of Herakles. He must report back to his aristocratic boss Hekataios of Miletus about what he finds beyond them. The ancient sages say they form the end of the world, that there is only a river outside them, called Ocean, and that it is filled with dangerous mists and misasmas. Are they right?
The task is easier to plan than accomplish. The obstacles Dubb faces go beyond normal navigational ones and are enough to make any man question the gods. Troublesome passengers clog the gangways of The Delphis. His skipper's rising terror as they move closer to their goal makes the man increasingly irrational; he even drowns a horse to placate Poseidon. As if this is not enough to deal with, Dubb discoveres that a vicious campaign of violence and sabotage is directed against The Delphis by spiteful Carthaginians who fear for the security of their lucrative shipping lanes.
All these problems, combined with Dubb's own fears and self doubt, threaten to sink the expedition.
When, eventually, The Delphis enters the treacherous waters of the Pillars, with the tantalising prospect of River Ocean lying just beyond, Dubb frets about who will stand by his side, who remain true to him whatever the cost. Sailing stalwarts from the past remain loyal. So does Qart who has earned his place on board as the navigator's boy. What though of Nyptan, the healer, a woman so beautiful that Dubb feels unable even to breathe in her presence?
Under the navigator's hesitant leadership all these characters on The Delphis join forces and become a crew so powerful that they give the Carthaginians the slip, outmaneuver wind and tide and overcome their fears to discover new things about the world beyond the Pillars of Herakles.