Complex sentence: Marty arrives at the clock tower where Doc is making final preparations for the lightning strike, and tries to warn Doc of his impending 1985 murder in a letter, but Doc tears it up, fearing it will lead to altering the future.
Split into simpler sentences: Marty arrives at the clock tower, where Doc is making final preparations for the lightning strike. Doc discovers a letter Marty had written to warn him of his impending 1985 murder and tears it up, fearing it will lead to altering the future.