Notorious criminal Vadim seeks to make life unbearable for the authorities that have imprisoned him. When he engineers a full-scale riot it falls to the Musketeers to protect those whom he seeks to destroy. Will the Musketeers discover his plan in time and stop him from endangering the monarchy? And can the inexperienced d'Artagnan prove he is worthy of becoming a Musketeer or do his loyalties lie elsewhere?

When Aramis's old friend Marsac, a former musketeer, unexpectedly returns to Paris, the musketeers are thrown into turmoil, fully aware that they should report him for desertion. His appearance coincides with the royal visit of the king's beloved sister and her husband, the Duke of Savoy, whom the musketeers must protect.

Marsac believes he finally knows the truth behind the infamous massacre which killed his troop, and wants Aramis's help to prove it. Despite their misgivings, will the musketeers help and can they prevent Marsac exacting his own revenge?