Taekang Group's successor, Yoon Yi-jun, is suspected of murdering his secretary, but the group's power and lack of evidence impede the investigation. To find key evidence, special forces-turned-detective Kang Jae-hee applies to be his secretary, not as a woman but as a man! She manages to get the job, but Yi-jun doesn't let his guard down. Jae-hee risks getting exposed on her first day at work.

Kang Jae-hee's double life begins. She's Yoon Yi-jun's secretary by day and a detective by night. Yi-jun keeps testing her, but Jae-hee relentlessly keeps an eye on him and searches for the missing phone. She plans to infiltrate the room that's banned from entering, but Yi-jun doesn't let his guard down easily. Their relationship takes a strange turn through unexpected physical contact.

Yi-jun stays prickly but starts trusting Jae-hee. Jae-hee notices something suspicious about the milk Yi-jun was drinking and realizes he is being threatened. The investigating team thinks the perpetrator could be someone else. Jae-hee finally gets the chance to enter the room that's been banned from entering. She searches for the missing phone in the room but immediately faces danger.

The investigating team finds traces of herbicide in the milk and suspects that someone is after Yi-jun. Jae-hee keeps an eye on Yoon Na-jung, the ambitious second daughter of Taekang, and follows Yi-jun to the family dinner, which is full of tension. Chairman Yoon makes an unexpected decision, causing a stir. Jae-hee joins Yi-jun for a drink and her drinking habit starts coming out.

Jae-hee can't remember what happened the previous night and is anxious if Yi-jun is crabby because he got wind of her true identity while drunk. What's worse, Yi-jun's fiancée, Lee Si-a, returns from France. Yi-jun feels confused but stays cold toward Jae-hee, and Jae-hee is upset when Yi-jun goes to have dinner with Si-a. After much searching, Jae-hee finally gets hold of the key evidence.

Yi-jun kisses Jae-hee but insists that he sees Jae-hee as a little brother and continues to nitpick. Meanwhile, the investigating team summons Hye-jung as a witness, shocking Yi-jun and Si-hyun. To make matters worse, Chairman Yoon is putting pressure on Yi-jun as well. Overwhelmed by the pressure and confusion, Yi-jun pushes away Jae-hee, who eventually has enough and says she will quit.

Yi-jun acknowledges his feelings as unrequited love. Jae-hee returns as his secretary and is confused as Yi-jun is suddenly being nice to her. Si-a's presence makes them keep their distance from each other. Hye-jung gets involved in the investigation on Han Eun-young's death, and she reveals she is being threatened and followed. The culprit is on the move, and Yi-jun and Jae-hee fall into danger.

Yi-jun can no longer hide his feelings for Jae-hee, but Jae-hee must keep her identity hidden until the culprit is found. In order to quell the rumors about them, Jae-hee goes on a blind date with a female Taekang employee. Unable to hold back, Yi-jun confesses, but Jae-hee turns him down despite feeling the same way. On the way to the memorial service, they face their greatest danger yet.