Aya is a detective specializing in monsters. She is said to be the only "immortal" being in this world, and has lived for 962 years. One day, she is attacked and must travel through Europe to recover the body that was taken from her. Since she cannot move alone, she is usually carried in a bird cage by her maid Shizuku Hasei. She strikes a deal with Tsugaru Shinuchi to extend his life in exchange for his help in finding her body.