A book of The d'Artagnan Romances, it is a sequel to The Three Musketeers (1844) and precedes the 1847–1850 novel The Vicomte de Bragelonne (which includes the sub-plot Man in the Iron Mask).
The novel follows events in France during the Fronde, during the childhood reign of Louis XIV, and in England near the end of the English Civil War, leading up to the victory of Oliver Cromwell and the execution of King Charles I.
Through the words of the main characters, particularly Athos, Dumas comes out on the side of the monarchy in general, or at least the text often praises the idea of benevolent royalty.
D'Artagnan, who seemed to have a promising career ahead of him at the end of The Three Musketeers, has for twenty years remained a lieutenant, and seems unlikely to progress despite his ambition and the debt the Queen owes to him.
Though Rochefort offers his services to Mazarin, he immediately refuses to watch over the imprisoned Duc de Beaufort, his personal friend.
Athos, the Comte de la Fère, had returned to his estate near Blois; Porthos, Monsieur du Vallon, had married a rich lawyer's widow; and Aramis became a priest, the Abbé d'Herblay.
D'Artagnan leaves but waits in hiding, suspecting that Aramis is both the Frondeur who had been sought earlier and the lover of Madame de Longueville.
The scene then changes, to focus on the Duc de Beaufort, Mazarin's prisoner at Vincennes, who finds a new jailer, Athos' servant, the silent Grimaud.
Using messages passed to Rochefort using tennis balls, they arrange to have a meal on Whitsuntide, to which La Ramée, second in command of the prison, is invited.
In a dramatic scene, the four friends come close to fighting each other, yet stop, reconcile, and renew their vows of friendship, which they value higher than any political alliances.
The monk reveals himself as her son, John Francis de Winter, who calls himself Mordaunt after Charles I stripped him of all his titles.
Grimaud, who is travelling to join Raoul, arrives to the inn just in time to learn, from the dying man, about the Mordaunt's existence and rushes back to warn Athos.
d´Artagnan and Porthos help Queen Anne of Austria, the young Louis XIV and Mazarin escape Paris after its citizens finally start a rebellion.
The champion of the French populace and parliament, Pierre Broussel, is arrested, but then released when it becomes clear that his imprisonment has only served to stir the crowd up worse.
Immediately after this, he contrives for all of the royal household to escape from Paris anyway, bluffing his way past Planchet at the gates (the two men retain their friendship despite their differing allegiances in this conflict).
Departing for England, Mordaunt, who has been following Lord de Winter, sees him with Athos and Aramis and learns that they were participants of his mother's makeshift trial and execution twenty years ago.
Mordaunt, a favorite of Cromwell's, murders his uncle, Lord de Winter, Milady's brother-in-law, and attempts to lay hands on Athos and Aramis, when the duo are the last men to defend Charles I.
In the end, all their plans fail and Mordaunt turns to regicide, executing King Charles I after d'Artagnan and the three former Musketeers have kidnapped London's official executioner in order to prevent this.
Unfortunately for him, the Musketeers' servants had earlier discovered the explosives on board, roused their masters, and contrived to steal the only lifeboat before the ship can blow up, leaving Mordaunt aboard.
In 1967, the BBC aired a 16-episode television adaptation by Alexander Baron titled The Further Adventures of the Musketeers, based on Twenty Years After,[3] with Joss Ackland as d'Artagnan, Jeremy Young as Athos, Brian Blessed as Porthos and John Woodvine as Aramis.