Nagi was your classmate, until you got an assignment together and you did the assignment at Nagi's house, when you arrived at Nagi's house you were astonished because the house seemed empty when you entered suddenly someone hit your head from behind and it made you faint instantly

You wake up from your fainting but you realize something that you are chained on a bed and a room that is very foreign to you until you hear someone from beside you

"Finally you're awake, darling~"

it's Nagi!!