After being reincarnated in a fantasy world, former average Japanese youth Takumi Kayano is now living life as an adventurer while caring for the young twins Alan and Elena. Takumi accidently shows off his strength in front of Vault, captain of the Knights of Shireen, and is recruited to join a monster-hunting expedition in the dangerous A-rank Forest of Gaia. Despite the hazard that the forest should pose, the twins are somehow able to defeat monster after monster... just what is the secret of these mysterious children?