Y/n is the only daughter of Mr. Kim and the beloved baby sister of her brothers; her life has been pretty normal and she's happy with it, until one fine day she receives a creepy love letter from a book that Namjoon, her childhood friend and first love, recommended. The immediate suspect is Namjoon. But y/n must be utterly sure to accuse Namjoon and if he's not the admirer, it might cost her their friendship. Pushing those thoughts away, she chooses to ignore the signs which tell her that something is fishy but as time passes, it turns out that every letter she gets is getting creepier than the first. So is Namjoon the secret admirer? Or is it just a coincidence? Will y/n ever catch the secret admirer like this? Or the creepy texts from the admirer will turn into something even horrible?