Complex sentence: He's followed by another visitor on the island, an ornithologist named John Lemmon (Rupert Everett) who goes through the loop with him who transformed himself into Dr. Golan and reveals that she is Mr. Barron.
Split into simpler sentences: He's followed by another visitor on the island, an ornithologist named John Lemmon (Rupert Everett) who goes through the loop with him. The birdwatcher transforms himself into Dr. Golan and reveals that she is Mr. Barron.