9th Sentinel Sisters blends a compact sci‑fi story with fast, arcade‑style action. The premise is simple and evocative: an alien egg buried in the earth hatches after millennia, spawning swarms that annihilate the surface. Humanity retreats into underground Arks and builds two desperate weapons to reclaim the planet - a manned war machine called "Father" and the cloned combat girls who pilot it. The setting is a burned‑out, near‑future wasteland and the mood fits the brief, gritty campaign to take back the land.