Complex sentence: Mary gets locked in the laboratory one night when she spies on the new assistant, Mr. Edward Hyde, what is going on in the laboratory and hides but Mr. Hyde is not as disapproving as she imagines he might be and lets her out.
Split into simpler sentences: Mary gets locked in the laboratory one night and spies on the new assistant, Mr. Edward Hyde, as to what is going on. She is found out but but Mr. Hyde is not as disapproving as she imagines he might be and lets her out.