Complex sentence: The motion picture is the first theatrical feature film based on the DC Comics character Superman; although two live-action Superman films starring Kirk Alyn and Noel Neill had already been shown in cinemas, they had appeared in a serial format.
Split into simpler sentences: The motion picture is the first theatrical feature film based on the DC Comics character Superman. Two live-action Superman films had already been shown in cinemas, starring Kirk Alyn and Noel Neill; they had appeared in a serial format.