Dan finally quits his job and leaves the apartment for good. He takes his grandma to the beach where she spent her childhood and wants to see that beach one last time. 
Before leaving the apartment, Dan leaves a thank you and apology letter. He doesn't wait for Jaekyung to say the goodbyes to his face. I personally loved this part. 
Jaekyung was having hard time accepting the fact that Dan has left him. But when he sees the grandma's wedding cabinet being tossed away and his empty apartment, that's when he realised that sh** has hit the fan and he fu**ed up. 
That's where the season 1 ends.