D'Artagnan having discovered that the girl he has rescued on his way to Paris is none other than the Queen's confidante, Constance, loses little time in becoming better acquainted. The Queen has a secret love affair with the Duke of Buckingham and as a token of her love, she gives him a set of twelve diamond studs. Richelieu's spy, Milady, discovers this and at once reports it to the Cardinal. He sends Milady to steal the studs and persuades the King to give a state ball and ask the Queen to wear the diamond studs, which he does. As soon as she hears this request she writes a note to Buckingham, but finds she has no messenger whom she can trust. Here Constance comes to her aid. The Queen gives her the note and also her handkerchief as a token and she leaves to find D'Artagnan. He is not far away, so she tells him his mission, gives him the note and handkerchief and bids him God-speed. Richelieu's spy has overheard their plans and hurries to report the matter to the Cardinal. He sends the spy out on the road ahead of D'Artagnan with instructions to prevent his reaching Buckingham. D'Artagnan in the meantime has confided to his comrades that he is on a dangerous mission and all three decide to accompany him. The spy manages to leave D'Artagnan's three friends disabled, but our hero arrives safely at Calais, where he finds the port has been closed. Buckingham's boat is about to lift anchor. He forces the Captain of the port to have him rowed out to the ship, where he meets Buckingham and finds that Milady is also on board. Milady manages to cut off two of the diamond studs and hurrying out of the cabin jumps into D'Artagnan's boat, and is rowed ashore, realizing that Milady has taken them they hastily call for a boat to go ashore, but Milady has taken the last one, so there is no way but to swim. Taking two valuable studs from Buckingham to replace the stolen ones, D'Artagnan leaps through the port and swims ashore. He wins the race to Paris, arriving in time to have the two studs set and delivers the twelve intact to the Queen, who generously rewards him by giving him a valuable ring and also his heart's desire, Constance.