The I.O.U. is a company who works secretly, providing justice using methods which may or may not break the legal laws. 25 years after it was founded, The I.O.U. recruits eight different girls, insisting that they owe the company and to pay for their debts they must become the latest unit named: Oh My Girl.

Hyojung is an innocent girl who has led most of her life digging up ginseng or something and becomes the leader of the group. JinE is a girl with a more secretive background, labeled as the strong man of the group. Mimi is a delinquet who has no permanent home and whose specialty is driving almost any vehicle. YooA is a talented circus kid who gets left behind by her troupe. Seunghee is a trouble child who was raised by several foster families. She manages to get up to any type of mischief possible, dubbed as a "disaster" and is a master of creating diversions. Jiho is a straight-laced, serious kid who was raised by a veteran soldier and has been brainwashed to be the perfect soldier. Binnie is a smarty pants who happens to run really fast yet has a past she's still quite traumatized by. Arin is what the company calls a "tiny fairy".

Will this team of oddballs be able to work together and fight crime?