Spoilers pls

SPOILER! (incl. end)
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...this basically is how it looks. They knew each other before. Dr Han was Hyuk's first love, but he just disregarded him after some kisses, as being caught up in the moment, that it's not real love, etc. Then he drove of on his motorbike into the sunset...
and tadaaa! we're in the present...
hyuk get upset and they fight...but it's a happy end...in the end.. :D

Thank you ^^