Complex sentence: One by one, the others remain behind to hold off their enemies, and only D'Artagnan reaches Calais, where he discovers that Lady de Winter has seduced Buckingham and gotten the diamonds.
Split into simpler sentences: One by one, the others remain behind to hold off their enemies, and only D'Artagnan reaches Calais. He discovers that Lady de Winter has seduced Buckingham and gotten the diamonds.