3 so-called-losers that dream to do the ABC (A= holding hands B= kissing C= NC-17) with a girl unexpectedly find out that quiet and handsome boy in their class named Ghang Han-gyul was secretly going out with a hot girl and then got dumped. Using this as a bait they pull him into their own world of peeking and dreaming about girls.
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Three boys who are desperately struggling to "make" a girlfriend in their sad adolescent lives. They then realize that their classmate, Ghang Han-Gyul - the manly, serious, quiet person of their class - has been secretly going out with a beauty for 100 days* (about 3 months).This triggers the friendship between the manly, serious Han-Gyul and the three dateless wonders.

* A 100 day celebration of a relationship is seen as very significant.

(taken from Mangaupdates)