The Third in Christian Cameron's Long War series, Poseidon's Spear picks up directly where Marathon left off, with Arimnestos of Plataea returning home after the battle of Marathon to discover his wife has died in childbirth. Despondant and suicidal, Ari throws himself off of a cliff, only to be rescued and immediately enslaved as a rower on a Carthaginian ship.

Unlike the previous two books, which both worked as coming-of-age stories, Poseidon's Spear works to show much more of the Ancient world from the view of a grown man. Without going into all the various twists and turns of the story, Ari ends up spending time in Africa, Spain, France, and Britain, and much of the book has him working as a tradesman rather than a warrior.

In the end, although the book moved away from the depiction of battles in the Persian war that the first two books focused on, it fleshed our Ari's world, introduced a number of new characters and felt like a really great break before moving back into the main thrust of the series.