Complex sentence: The trope of the white savior has an extensive history in the cinema of the United States, wherein he or she is portrayed as a messianic figure who learns something about himself or herself in the course of rescuing the non-white characters.
Split into simpler sentences: The trope of the white savior has an extensive history in the cinema of the United States. In the narrative, the white savior is portrayed as a messianic figure who often learns something about himself or herself in the process of rescuing the non-white characters.