Complex sentence: Stoddard's story flashes back several decades to his arrival in Shinbone via stagecoach, which was robbed by a gang of outlaws led by gunfighter Liberty Valance (Lee Marvin).
Split into simpler sentences: Stoddard's story flashes back several decades to his arrival in Shinbone. His stagecoach is robbed by a gang of outlaws led by gunfighter Liberty Valance (Lee Marvin).