The Enigma of Arrival.

Long, the main character, spends his youth wandering with his three best friends from high school. With the emergence of a woman, Dongdong, whom they all fall in love with, their friendship begins to fray. One day, Dongdong suddenly disappears in an ambiguous situation, and the friendship ends. After many years, the friends meet again. However, just as youth cannot return, they can only confirm that their friendship cannot be restored.