This is not a light hearted romance comedy but a mature romance drama dealing with the topic of adultery (unknowingly by the female protagonist, who stops the relationship once she knows he has someone else), it is dealt in a very very interesting, tasteful and smart way.
So it is not actually about adultery, but about love..

For the FL, it is about healing and moving on after a 2-year relationship to the 2nd ML, who secretly gets committed to someone else (sick) during the course of their relationship , not because of love but because of greed…
it makes sense that both relationships of the second ML remain secret to the world, the one with the FL because they are coworkers, and the other because the GrandFather is opposed to it, and owns the Company they work at. Also, he doesn’t love her… We are told that he kind of had not choice in agreeing to marry the CEO’s grand daughter, but plans to keep the FL, who he really loves, as his mistress after marriage, hence the drama tittle, he hoped « she would never know »…because he knows she loves him deeply and he counts on it when she will learn the truth not to break up.

The best part of the story for me is about the male protagonist, played by Rowoon,: his story arc and genuine personality in the drama makes it one of a kind, extremely well written, directed, insightful, interesting, and … swoon worthy. As rowoon says in the drama, he wants to make the FL live the most beautiful love there is… and that’s exactly what he does throughout. He offers her an unconditional pure mature love, which not everybody is able to receive , especially someone with scars and who hasn’t been loved by her parents…
,.
We also understand that without the support of Rowoon, who told her the truth about her boyfriend, she might have compromised her integrity and accepted the situation once known to her, that’s how much she loved him in the beginning.
But ultimately, she has a strong character and manages to overcome her feelings for her ex boyfriend. And that is quite interesting, because usually, the mistress would keep hoping and waiting for her lover to choose her…

On a lighter side, Rowoon character in the drama is the perfect boyfriend material, absolutely kind, caring, charming, He brings light to the drama, like the second leads arcs., the ones of his two great sisters. What a treat to watch them interact with each other.

Rowoon wins the FL over in the end thanks to his pure personality. dedication and unwavering supporting unconditional love,

All in all, not the classic rom com,
Extremely well treated difficult topic
Very smart and well directed
With meaningful dialogs that make you reflect
Doesn’t fall into adultery unpleasant pitfalls, excusing an unhealthy / amoral relationship under the pretence of love…

And a great positive ending lasting a full episode
A beautiful love story
Rowoon is decidedly a promising actor