Grieving the loss of a best friend she couldn't protect, an ex-bodyguard sets out to fulfill her dear friend's last wish: sweet revenge.

At work, she's a renowned assassin. At home, she's a single mom to a teenage daughter. Killing? That's easy. It's parenting that's the hard part.

'Monster Cop' Ma Seok-do investigates an illegal online gambling business led by a former STS Baek and an IT genius CEO Chang. Ma proposes an unexpected alliance to Jang and begins hunting down the criminals.