"The scarlet tattoo marked him as one who had never been born. It marked him as a Second." In the age of inventors, Seconds are beings who were never meant to exist. Split personalities that have gained physical form, Seconds possess thoughts, feelings, and bodies of their own but are seen as a nuisance to the residents of the mechanized city of St. Laran for their taboo nature and the pain they cause to those who spawn them. Eighteen-year-old heiress Tide Yondo is rare among her peers, in that she has little exposure to the beings known as Seconds. Kept sheltered from the public at the top of the city's highest apartment complex, Tide decides she's had enough and seeks to earn her scrap huntress license so that she can begin moonlighting as a scrap climber, fulfilling ruin-exploration quests for the lower levels of the city. When, on the day of her huntress licensing test, Tide meets her first Second-a fanboy named Rye-she quickly realizes he's nothing like the 'despair in a suit of flesh' her friends warned her about. Rye is fun and cute and seems as solid as any other person she's met, save for the red tattoo on his neck identifying him as one who shouldn't exist. Tide and Rye form a fast friendship over their love of scrap climbing, and because it's illegal for Seconds to leave the city, Tide shares her experiences with Rye so that he can live vicariously through her climbs. But as forbidden feelings begin developing between the huntress and Second, both start to question their chance meeting. Tide frequently blacks out and claims to have a serpentine demon after her, Rye feels like he may have met Tide before being split from his primary personality, and there seem to be outside forces trying to drive them apart. Through steam-filled ruins and dangerous hunts, Tide and Rye will slowly mend the pieces of their fractured pasts as they work together to discover the secrets of the soul and what it means to be alive.