Recycled Brain delves into a world where humanity has unlocked the ability to create custom life forms, including Pegasus or Dragon. The story follows Hayate Yamano, a guide in a unique "theme park" where visitors can interact with fantastical creatures. However, the idyllic setting is shattered when a group of cultists launches a surprise attack, sparking a conflict that threatens civilization itself. As tensions rise and hidden truths come to light, Hayate finds himself at the forefront of a moral and existential crisis.