Four men with complicated love lives refuse to admit they're getting older, and attempt to prove that forty is the new thirty!
Yi-soo likes Tae-san but he goes on a date with Sae-ra, her roommate.
Meari's meddling brings about some strange consequences for the singles. Instead of coming clean, Yi-soo chooses to lie and profess her love to Do-jin for Tae-san's benefit.
Yi-soo refuses to answer her phone when Do-jin calls. Se-ra believes Yi-soo either loves Do-jin or hates him.
After Jung-rok swallows his wedding ring, Yoon lets it slip to Do-jin that he likes Maeri. Se-ra finds a copy of Tae-san's yearbook photo in Yi-soo's possession and figures out that they both like Tae-san.
Se-ra has figured out that Yi-soo has feelings for Tae-san. She hasn't yet said that she's willing to give him up, but she wants Yi-soo to admit her feelings.
Consumed with jealousy, Se-ra reveals the truth about Yi-soo's crush to Tae-san. However, things start to heat up between Do-jin and Yi-soo.
Tae-san lets Yi-soo know that he really likes Se-ra. After Se-ra confronts Yi-soo about her feelings for Tae-san, Yi-soo decides that she wouldn't mind dating younger guys.
Tae-san has an issue with Yoon having feelings for his sister. However, he is put into an even worse position when Meari confesses her requited love for Yoon.
Do-jin is happy to see Yi-soo wearing his gift until he thinks she's falsely declaring her love for him again. Unbeknownst to both of them, Yi-soo is actually falling for him.
After misunderstanding Yi-soo's intentions for him, Do-jin has decided not to see Yi-soo again. However, Yi-soo realizes her feelings for him and tries to straighten everything out.
Do-jin and Yi-soo manage to clear the air, but Do-jin wants to balance out the relationship. In order to even the score, Do-jin creates a checklist of one-sided tasks for Yi-soo to complete.
Tae-san and Se-ra find themselves at odds and end up getting into the biggest fight of their relationship.
Tae-san continues to struggle with the idea of Meari and Yoon's age difference and he is unwilling to accept their love for each other. In the meantime, the guys can no longer ignore Colin and have to make a decision.
As each person wonders about Colin's paternity, each couple experiences their own set of misunderstandings and unnecessary accusations are thrown.
Yi Soo is still hurting but she's in the mood for revenge. Jung-rok, Tae-san and Yoon are adjusting to having a new family member but end up accidentally embarrassing him.
Yi-soo is struggling to accept the reality of Do-jin's relationship with Colin, but in time she comes to a decision to fight for her relationship. In a strange turn of events, Colin is sent to Yi-soo's school and is placed in her homeroom.
After all of his unsuccessful attempts to push her away, Do-jin is open to the possibility of having a relationship with Yi-soo. The other couples continue to move forward.
Yoon is finally able to confess his feelings for Maeri. Do-jin is able to impress Yi-soo's co-workers by treating them to dinner.
Although he was initially angry, Tae-san is willing to accept his sister's relationship under one big condition.
