Complex sentence: Allen realizes she was the young mermaid he had met so long before, and although Madison warns him that if he comes to live in the sea he can't return, he jumps into the water after her and they elude their pursuers.
Split into simpler sentences: Allen realizes she was the young mermaid he had met so long before. Although Madison warns him that if he comes to live in the sea he can't return, he jumps into the water after her and they elude their pursuers.