Because he's a Beast.

A skilled employee at a prestigious company hides a secret identity: he is also the family's ninja bodyguard. His master, however, behaves like a naive princess, constantly clinging to him at work and teasing him with offers to become her husband. She takes every chance to get close, while he internally protests, thinking he is not her sex slave, yet he cannot resist her pleading expression and ends up sleeping with her night after night.

Their secret relationship faces a new threat as the higher-ups begin to quietly take action to separate them. This modern tale of master and servant explores the tension between duty and desire, as the ninja struggles to maintain his boundaries while succumbing to his feelings for the princess.