At the end of the movie Back to the future. Marty Mcfly is left by Doc Brown in his time train , Marty realizes he has got to find his friend, he finds a machine that sends him across the universe and drops him in the Tardis from doctor who, The Doctor and Marty try to find doctor brown who gives them a tour of the new time train. Which sends them bouncing between dimensions. Picking up friends like Twilight Sparkle and Ditzy Doo to help save the universe from collapsing within its self.