Tsutomu Senkawa, a 17-year-old Japanese student with black hair, brown eyes, and a slender frame, stands at 155 cm. His ordinary life shifts abruptly when Birdy Cephon Altera, an Altan Federation officer hunting fugitive alien Geega, accidentally targets him during a pursuit. A fatal misunderstanding leaves Tsutomu gravely injured, prompting Birdy to fuse their bodies to save him. This merger forces them into a shared existence, with control over their combined form shifting unpredictably—often bypassing Tsutomu’s will. Now entangled in Birdy’s intergalactic mission, he struggles to reconcile routine school life with clandestine dangers, relying on resilience and adaptability to navigate the chaos of their intertwined fates.