Complex sentence: In the original "BTTF" novel, the character Marty McFly decides to take in a movie when he finds himself transported back to 1955 and chooses "The Atomic Kid" as opposed to "Cattle Queen of Montana" at the nearby Essex Theater.
Split into simpler sentences: In the original novel, the Marty McFly character decides to take in a movie when he finds himself transported back to 1955. He chooses to see "The Atomic Kid", as opposed to "Cattle Queen of Montana", which is being shown at the nearby Essex Theater.