Transmigrated to another world and forced to live as "Karen," Ichika is frustrated about her sister, the saint who stole her identity. But when she saves her fiancée Noah from a curse, it's revealed that she is a saint as well! Now, armed with knowledge of the magic she wields, Ichika takes on a job to purify a river together with Noah. As the two grow closer even through bickering, Ichika discovers a whole new power residing within her...