In the human world, the summoner and the servant ( from other world ) forms a pact where they are bound together. The summoner may call their servant to engage a battle with other summoner and the servant must protect their master at all cost.

When Olivia Flynn, a heartbroken 16 years old Swiss, moves to Japan, she comes across a famous patisserie that consoles her from her unfortunate moment. What Olivia doesn't know is that the patisserie guy consoling her is ACTUALLY her servant, and meeting him sets off a chain of events that turns Olivia's life upside down.