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.