Once there were a couple named Sujal and Shristi. On valentines day Sujal approached Shristi for coffee. They were having romantic moments, spending together, and drinking alcohols . After celeberation, They went their own home.  While going shristi had an car accident with Biker. Because of his fault she also died on the spot. As soon as boy finds the news about girl he started crying and he was in stress. He didnt eat anything and he became weak. As time passed by, sujal started healing himself , but still didn't forget her. Anyhow he want her back, so started on project. Project of time machine. He made a tachyon particles . Its a particle which has higher speed than light. He made a machine using this particle and he used  time dilation method. After years and years passed by finally his project become successfull. Now he become an old man. So any how he want her back so he travelled back in time and he wanted to save her in that accident . So he set the time and went to that location  and he was little bit late so he lent a vehicle from someone . She was already in car ,going home. And Sujal was in hurry  and he crossed his speed limit and he also had an bike accident with car. 

And do u know who was in car?

His girlfriend. We can conclude that the he was the cause of her death.