The Doctor, Ian, Barbara and Susan arrive in what they believe to be England. However, it is soon clear that they have travelled back into Earth history yet again, this time to 18th Century France. They have arrived in a wood twelve kilometres from Paris, then venture to a nearby farmhouse which seems to have seen better days. When they venture inside they find the farmhouse is being used as a staging post in an escape chain for counter-revolutionaries and contains clothes – some of which are put to good use by the travellers - and fake papers, some of which bear the signature of Robespierre, the chief orchestrator of government during the Reign of Terror. Before long they are found by two counter-revolutionaries, D'Argenson and Rouvray, who knock the Doctor unconscious and hold the others at gunpoint. The impasse is ended when a band of revolutionary soldiers surrounds the house and demands their collective surrender. Both D'Argenson and Rouvray are killed during the siege, but only after they have worked out that there must be a traitor in their escape chain. The soldiers now enter the house and capture Ian, Barbara, and Susan, who will be marched to Paris and the guillotine. The parting action of the soldiers is to set fire to the farmhouse – unaware there remains one person inside.
The Doctor awakes the next morning suffering from exhaustion and smoke inhalation. He was saved from the blaze by a young boy, who tells him his friends have been taken to the Conciergerie Prison in Paris. He sets off after them and, after a brief period press-ganged into a road mending crew for lack of papers, makes some speed in the twelve kilometre journey.
Things are looking grim for Ian, Barbara and Susan. They are all sentenced to death as traitors without a chance to speak in their own defence, and are all promised the guillotine for their crimes. Back at the Conciergerie, Ian is confined in one cell, while the women are taken to another. Ian's cellmate is an English prisoner named Webster who only lives long enough to tell him there is another English spy, James Stirling, highly placed in the French Government who is now being recalled to England. It was Webster's job to find him and he only knows that Stirling can be found through Jules Renan at the sign of "Le Chien Gris". Once Webster is dead a government official named Lemaitre arrives and probes any conversation between Ian and the dead man. Lemaitre also crosses Ian's name off the execution list.
Susan and Barbara are less fortunate. Confined to the filthiest cell in the jail when Barbara refused the advances of their drunken jailer, they are soon taken on a tumbrel to the guillotine. Luckily for them their transport is hijacked by two men, Jules and Jean, who spirit them back to their safe house. Susan is, however, somewhat ill from her exposure to damp conditions. They are told they will be given food and a place to rest, and then they will be smuggled out of France in the escape chain, but Barbara is nervous about leaving France without the Doctor and Ian. Jules and Jean reassure her they will try to reunite the four travellers, and they all exchange tales of the fate of D'Argenson and Rouvray, whom it seems were part of the same escape chain. Another counter-revolutionary, Leon Colbert, arrives and joins their company, immediately striking up a romantic liaison with Barbara. The Doctor has finally reached Paris and exchanges his clothes and Roman ring for the costume of a Regional Officer of the Provinces plus some parchment and writing materials. In this guise he heads for the Conciergerie, but by the time he gets there all three of his companions have gone. Ian has successfully stolen the key to his cell from the incompetent jailer and managed to get away, not least because Lemaitre seems to have rendered the jailer unconscious to aid his escape. The Doctor ascertains what has happened to his friends and is about to leave when Lemaitre arrives and insists he accompany him to visit First Deputy Robespierre to report on his province. They are soon taken to an audience with "The Tyrant of France" himself, who appears as both a zealot and a psychopath with his constant talk of needing to increase the pace of execution. Little the Doctor can say to the contrary seems to have any sway, and he departs angrily.
Ian follows Webster's words and hunts out Jules Renan, who turns out to be the man sheltering Barbara and Susan, who remains critically ill in bed. Ian shares the last words of Webster with his host and they both agree that Stirling must be found and returned to England to help end the war with France which is helping to create the climate of fear that is sustaining Robespierre and his tyranny. Barbara meanwhile takes Susan to a local physician who wastes no time in reporting them as escaped prisoners to the authorities and they are seized once more before the revolutionary police. Things take a turn for the worse for Ian too when he goes to meet Leon in secret – only to find he is the mole in the escape chain and there are armed troops waiting for him. Leon Colbert is desperate to find out what Webster said to him, but Ian is very guarded in his comments.
The Doctor has returned to the Conciergerie where two people are waiting to see him. The first is Lemaitre, who reports that Robespierre wishes to see him again the following day. The second is the tailor who sold the Doctor his clothes and has reported his suspicions of him to the jailer and Lemaitre. The latter confiscates the ring and dismisses the tailor, saying he will deal with the situation. Lemaitre ensures the Doctor spends the night in the Conciergerie in order that he remains in Paris for his second audience with Robespierre. He is still there when Barbara and Susan are brought in as prisoners, and effects a reunion with his friends without raising too much of an alarm. With Susan to weak to be moved, he engineers Barbara's release first on the pretext that she can be trailed to lead the security forces to the core of the escape chain.
Jules Renan has meanwhile rescued Ian, killing the traitor Leon Colbert in the process. Ian has been wounded but yet they both know they need to get their friends freed from the Conciergerie somehow. They return to Jules' house and are stunned to meet Barbara there, released on the authority of the Doctor of whom Jules knows nothing. Barbara is hurt and saddened by the death of Colbert, seeing good in all sides during the revolution.
Robespierre's mental state is deteriorating and he suspects his deputy, Paul Barras, is conspiring against him in the Convention. He asks Lemaitre to track Barras the following day to a secret assignation outside the city. When Lemaitre heads back to the Conciergerie it is to confront the Doctor, whom he unmasks in private as an impostor. Lemaitre now insists the Doctor help him find Jules Renan's house and expose the spy ring. With Susan held in the prison as a hostage, the Doctor takes him to the very place, much to the concern of Ian, Barbara and Jules. Once there Lemaitre reveals that he is in fact James Stirling. In response, Ian relays Webster's message that Stirling should return to England immediately. The spy agrees but presses Ian for more detail on Webster's last hours. When Ian recalls the words "Barras, meeting, 'The Sinking Ship'", Stirling recalls his own conversation with Robespierre and the inn on the Calais Road, and they realise that is where the conspiracy against the First Deputy will take place. Jules, Ian and Barbara head to the inn and there overhear Barras conspire with a young general, Napoleon Bonaparte, in the indictment and overthrow of Robespierre. Barras seeks to persuade the young general to take the mantle of leadership when it comes as one of three Consuls. Napoleon urges Barras to topple Robespierre, but warns him that if this fails to happen he will deny this meeting ever took place.
The following day Stirling fulfils a pledge to the Doctor and arranges Susan's release from prison. Speed is of the essence as the coup against Robespierre has begun, and the tyrant has been badly wounded before being seized himself and sent to the Conciergerie. The Doctor and Barbara reach the prison where he outwits the jailer one more time, ensuring Susan is freed to escape with them. As they leave by one door, the bleeding Robespierre is brought in by another, his body broken and his rule ended. He will be guillotined himself soon.
The escape chain now demonstrates itself to best effect and smuggles several people out of Paris. Stirling will head for Calais and thence to England; Jules and Jean will lie low as they measure the future now that Robespierre has fallen; and the Doctor and his companions are keen to return to the TARDIS in the wood near Paris, reflecting on another brush with history and their role within it.