The Iron Horse is a massive steam-powered locomotive manned by a crew of warriors, miscreants, and genetic anomalies. Socrates, the train's commander, is an autonomous steam-powered gorilla with memories and knowledge reaching back more than a thousand years. Their mission is to locate a new source of starfall, the rare element that powers the ancient city of Sanctuary. Their civilization needs starfall, but for Socrates the matter is personal. Without it, he will die.

After a brief respite on the shores of the Forgotten Sea, the crew readies for their next great adventure. This time, the tracks will take them somewhere entirely different. The only way across the great sea is under it. But as the Iron Horse plunges into the icy waters, something goes wrong. The train has been sabotaged, and to make matters worse, their fuel supply has been stolen.

Trapped at the bottom of the sea, faced with the choice between a slow death by suffocation or a fast one in the crushing black waters, Socrates makes a last ditch effort to save the crew. He believes there is a cavern ahead, if they can only make it that far... But even if they survive, the crew may face an even greater peril from a completely unexpected new enemy.