Complex sentence: In the chaos, Carlyle believes Wheeler to be in the burning building and races to rescue her; when Wheeler emerges unharmed on her own, Barnum heads into the burning building to save Carlyle.
Split into simpler sentences: In the chaos, Carlyle believes Wheeler to be in the burning building and races to rescue her. When Wheeler emerges unharmed on her own, Barnum heads into the burning building to save Carlyle.