Marc is what happens when you survive the apocalypse but the apocalypse survives in you. The show doesn't flinch from what that costs, it shows you a man breaking apart piece by piece, made worse by the fact that you understand why he's breaking. Bai Yuekui knows his origin story before he does, which means she's been watching him destroy himself, and that quiet knowledge between them is heavier than any action sequence the show could throw at you.
Marc returns as something other than human, methodically destroying the lighthouses that once promised salvation. Bai Yuekui recruits him for a purpose neither fully understands, pulling him toward answers buried in their shared past. When ground survivors finally reach the floating stations, the truth shatters: humanity's last refuges are prisons, not sanctuaries. The world below pulses with hostile life, a sprawling organism that thinks and hungers. Those who've abandoned the lighthouses must awaken dormant powers within themselves to survive contact with this alien ecosystem. Each discovery about the cataclysm that ended civilization reframes what Marc and Yuekui thought they knew about themselves and their mission. An ancient intelligence stirs in the lighthouse's core. Fractured factions of humanity now face impossible choices: retreat into false security or descend into a world that actively resists their existence. As catastrophe accelerates beyond anyone's control, former enemies and reluctant allies must navigate whether survival means finding refuge or becoming something entirely new.