In the slums of the sea-battered city, a young boy called Nathan Treeves lives with his parents, eking out a meagre existence by picking treasures from the Living Mud and the half-formed, short-lived creatures it spawns. Until one day his desperate mother sells him to the mysterious Master of Mordew.

Helliconia follows an eccentric orbit around a double-star system with a twenty-six-hundred-year cycle of very long seasons. As spring slowly breaks the brutally long winter, humans emerge from hiding and a long sequence of civilization and growth begins to repeat again, unbeknownst to the participants but watched by an orbiting satellite station, Avernus, created by Earth some centuries ago. Humans free themselves from slavery to the aboriginal Phagors, and religion and science flower and expand.

Once, Sammy Dominguez thought he knew how the world worked. The ugly things in his head—his uncle’s pathetic death, his parents’ mistrust, the twisted horrors he writes for the Internet—didn’t matter, because he and his girl, Ellery, were on track for the good life in this messed-up world. Then a car accident changed everything. Spiraling with grief and guilt, Sammy scrambles for distraction. He finds it in shock-value videos of gore and violence that terrified him as a child. When someone messages him a dark web link to footage of Ellery dying, he watches.

Ambassador Mahit Dzmare arrives in the center of the multi-system Teixcalaanli Empire only to discover that her predecessor, the previous ambassador from their small but fiercely independent mining station, has died. But no one will admit that his death wasn't an accident - or that Mahit might be next to die, during a time of political instability in the highest echelons of the imperial court. Now, Mahit must discover who is behind the murder, rescue herself, and save her station from Teixcalaan's unceasing expansion.

Brother Diaz has been summoned to the Sacred City, where he is certain a commendation and grand holy assignment awaits him. But his new flock is made up of unrepentant murderers, practitioners of ghastly magic, and outright monsters. The mission he is tasked with will require bloody measures from them all in order to achieve its righteous ends. Elves lurk at our borders and hunger for our flesh, while greedy princes care for nothing but their own ambitions and comfort. With a hellish journey before him, it's a good thing Brother Diaz has the devils on his side.