Renowned astronomer Marmion gathers the royal household to his impressive observatory in a transformed old fort to watch a solar eclipse. The Musketeers and Rochefort escort them but could never have predicted the twist in fate that ensues and the impossible choices that must be made.

Who is Marmion really and what is his game? The Musketeers are forced into the fight of their lives with dramatic consequences. Will they accept Milady's help despite their mistrust of her? It might be their only chance to save the king, queen and themselves.

Porthos uncovers the truth about his parentage and demands answers from his new-found father. As he learns more about his past, Porthos questions his place in the Musketeers regiment. However, the Musketeers have suspicions about Porthos' new family, wondering whether he should trust them. Can they save Porthos from the dangers that lie ahead, and where do his true loyalties lie?