the plot is basically following both their developing relationship/group project + uncovering the mystery behind the H100 drug (smiley face pill). it all connects when you think about the reason behind DREAMBOY’s disbandment +. jaeyoung’s ptsd! it’s actually a very interesting plot that comes to a full circle :) ofc we are still missing details, but if you’re confused abt anything i can help explain it maybe!
It kinda explains it with flashbacks, the fifth guy of the group which in the picture they often show is the last one on the left looking at jaeyoing started taking the h 100 drug and made jaeyoung take it one day when jawyoung followed him to a shady party. The drug makes it so that you can't feel any emotion anymore and makes bright light unbearable, eventually the records society found out and there are various possibilities such as they found jaeyoung drugged the day after / jaeyoung lost the ability to sing and perform on stage / the other guy disappeared etc this is still unknown, the result is that the records company made the group disband and hid the scandal. Another point is that the drug has been brought to the idols by one of the records company manager, which is probably also unknown. Another subplot is the fact that Seungmin has been investigating on the same drug, the drug comes from Brazil where he's born so we don't know the reasons why he's been investigating but there must be something connected in his past too. The effects of the drug are unknown so the fact that jaeyoung is on meds with the doctor is important because they could learn how to get rid of the effects but the Seungmin's girl friend and owner of the club just discovered that the doctor's patient might be jaeyoung. Lastly, in order to feel something you have to keep taking the drug, as the records manager explained it gives you a big shot of adrenaline so apparently the fifth member of the group is still taking it and enjoys making other people as miserable as him by making them take the drug (he fed it to the undercover policeman in the pool). So many things are still to be uncovered but it's starting to be clearer.

I already re-read it twice, I still don't understand anything ahahaha. I think I will let it marinate before rereading again. Ahaha