Engineer Meng Haonan takes his younger sister, Xie Mengran, camping by the sea. A bolt of purple lightning strikes Fontaine. Black flames spring from her brother's left hand, while his sister can pin people down with a glare. They possess two "Original Sin Authorities," both of which spiral out of control when their emotions run high. The Gardes put up wanted posters across the city, so the siblings hide in the underground slums, making ends meet by repairing machines and drawing quick sketches. They take a commission to investigate disappearances in the waterways and stumble onto a Sin Erosion conspiracy. Unable to stay in Fontaine, they flee to Sumeru and explore ruins, then head to Inazuma and take a slash from the Raiden Shogun. After stumbling their way through one fight after another, they reach the ruins of Khaenri'ah, where the Heavenly Principles confront them. The siblings clasp hands: We decide what this Original Sin means.
