
Spoiler
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Read the 1st page of ch 1.You can see Theo on a bike sitting behind Os.He also states something about the expression on his family members,that he hears his childhood friend's wrenching sob for the 1st time(you can see Danny literally stooping).Basically this scene is that of the ending where Theo kisses Os infront of everyone and decides to leave his home forever making everone sad especially Danny.
So,what I actually said is that the story actually starts with how it ends i.e them leaving but the whole manga is kind of a flashback as to what events led them to meet,fall in love and leave ch 1 http://www.mangago.zone/read-manga/rumspringa_no_joukei/hs/mpn_chapter-1950923/5/
The ending
http://www.mangago.zone/read-manga/rumspringa_no_joukei/raw/tr_chapter-0.1/pg-221/
http://www.mangago.zone/read-manga/rumspringa_no_joukei/raw/tr_chapter-0.1/pg-222/
This might end up in a bittersweet ending or not.