Complex sentence: The footprints turn out to be those of Mary's undisclosed true fiancé, Goyles, a Socialist agitator considered "an unsuitable match" by her family, who had secretly entered the grounds for a pre-arranged rendezvous at 3am, when the couple had intended to elope.
Split into simpler sentences: The footprints turn out to be those of Mary's undisclosed real fiancé, Goyles, a socialist agitator considered "an unsuitable match" by her family. He had secretly entered the grounds for a pre-arranged rendezvous at 3 am, when the couple had intended to elope.