I know, it's a very sad song. It took me so long to understand the story and now I can go through the whole plot easily. At first, it looks so boring to me and I don't even bother to find out but then I watched it again just interested in the song and I understood the whole story.

It started when both of the twins: Rin and Len had to be separated and Rin grows up to be an evil person. But when Len saw her again and he really loves his sister, he would do anything including being a Servant of Evil, which logically means him being the servant who would do anything the Evil wants.
So this one day, Len saw Miku Hatsune with Kaito and he fell for Miku and her adorable smile but Rin loves Kaito and for being jealous of their relationship, Rin told Len to kill Miku for her. Because of his love for his sister, he killed Miku reluctantly, this can be seen when he cried while stabbing the knife onto Miku.
So... truthfully this part is a bit... I don't know, I cannot interpret the story but it kinda looks like Miku is a princess and the royal guard, searched for the murderer. They finally know it's Rin and she's scared but Len is there and he disguised himself as Rin for they are both similar twins and he sacrificed himself for Rin.
At the end of the MV, Rin can be seen in her hoodies looking at Len who at the end, died.