It's set in a world ruled by Gods, with actual powers and control over the elements, where each family has to give up a child to be given to the Gods to keep the world working. Children are periodically collected from families, but I don't remember if the logistics are ever explained; I think it's one child per family generation or so, otherwise it wouldn't be sustainable, but I digress.
A bird family (the world is populated by animal people as well as humans) raised one of their sons, meant to be sacrificed, in a shed and without love or a name. Easier to give him up that way. But his small sister, Beatrice, loves him and gives him the name Pigeon. The story starts with him being collected to be a sacrificer, and the story unfolds from there. We learn that the children are given a glimpse of a happy life with food and comfort, only to have their life energy harvested with machines, into a concentrated liquid which the Gods then drink, extending their lives and increasing their powers. If I recall correctly, Pigeon manages to escape before he's harvested.
Soluna, the daughter of the chief Gods of the Pantheon, the Sun god and the Moon goddess, is headstrong and rebellious, and her disobedience lands her (unbeknownst to her parents) in a new batch of sacrificers, along with Beatrice, who has been collected as a sacrificer due to Pigeon's escape.
Even as her parents desperately search for her, Soluna is partially harvested, and her power ends up being consumed by none other than Pigeon.
Pigeon, with his new powers, wages war on the Gods, going into their lands one by one and fighting them, hoping to end the Sacrificer ritual.
Meanwhile, Soluna (who somehow also got infected with a venom-like parasite goo thing, I kind of got lost regarding that plot point) manages to escape with Beatrice.
The Gods are in disarray, and the Moon goddess abandons her role.
With so many of them dead or fleeing, the elements are out of control and the world is undergoing massive disasters.
Pigeon, with the help of a lesser God and his army (they were a mining/forging civilization, so they valued collaboration and work highly, which made them kind of communist-ish, but also empathic towards Pigeon's crusade), finally manages to defeat the Sun God, but nearly dies in the process. Soluna and Beatrice's own adventures led them to the same spot, so they all meet at this point.
There's a large time jump, and we find Pigeon and Soluna married and had two children, and are now the new rulers of the world. Without active God, and despite the efforts of Soluna using her (recovered) powers to keep things working, the world is in ruins. Beatrice, now a high priestess, joins a revolution against them, and war starts again. Pigeon's people have been working for years on a machine that will solve the world's natural crisis (or at least make things good enough that they can start rebuilding), but just as they activate it, and it seems to be working, the rebels attack and destroy it, dooming the world.
Soluna, weakened by years of keeping the world alive on her own, is dying. Pigeon refuses to let their children use their powers, feeling it would simply reinstate the old order. His last hope is, ironically, using the harvesting machines to extract enough life liquid to heal and empower Soluna, and hopefully try the machine again; he commits the sin and harvest a few subjects, and runs to give Soluna the liquid, just as the rebels break into their palace.
But Soluna is dead.
Beatrice sees her dead, and sees the children, her nephews, and leaves the palace with them. Pigeon finds Soluna dead, and tries to give her the liquid, but it doesn't work. We don't see him die, but obviously he'll die of sorrow next to Soluna.
In the end, Beatrice takes the children to what's left of the farm where she and Pigeon grew up, and asks them to use their powers to heal the world, which they do, causing nature to be revived across the land.
The End.
So, my reflection on all this is, what a curious take on the ideas of oppression and explotation of the common people.
Sure, it's a cruel thing that the Gods did, and they lived in luxury and self-congratulation while the people suffered poverty and sorrow from the sacrifices, but it worked, it kept the world alive.
The moment they were overthrown, the world began to decay, and the only genuine attempt to actually fix things without abusing or sacrificing anyone, Pigeon's machine, was thwarted by an angry mob of revolutionaries.
And like Pigeon feared, the solution in the end was to use his children, the new Gods, to heal the world. Sure, it ends with them looking upon a verdant land, but how long until they need to feed on sacrificers again to power themselves up and keep sustaining the world?