Complex sentence: In the end, Ruth and Carl discover the truth: that Mary Ainslie was engaged to Charles G. Winfield, Capt. of the Schooner Mary, but that he married Carl's mother, Abigail Weatherby instead.
Split into simpler sentences: In the end, Ruth and Carl discover the truth. That Mary Ainslie was engaged to Charles Winfield, Capt. of the Schooner Mary, but that he married Carl's mother, Abigail Weatherby instead.