A bell is ringing as a soldier brings word to General John Lambert that Lord Protector Oliver Cromwell is near death. As they go to Cromwell’s chambers, Lambert and the soldier discuss the storm that is raging and the great sea beast that has beached on the banks of the Thames, both of which are seen as portents of the Lord Protector’s passing and a dark future for England. Lambert despairs that Cromwell will appoint his son Richard as the new Lord Protector; Richard is “a fool and a weakling.” At Cromwell’s bedside, Lambert claims that Cromwell is not dying, but Cromwell insists that he is, and must name a successor. He surprises Lambert by passing over Richard and giving the rule of England to Lambert himself. As Cromwell declares this decision the result of a message from the Lord, a mysterious voice calls out “Lambert!” in exultation. Cromwell dies, and a new Lord Protector is proclaimed…

The Professor and Ace have arrived in London in 1998, and Ace insists that something is wrong with Covent Garden; the streets are two clean, and there are anti-gravity lifts everywhere. The Professor brushes it off as the result of time passing, but is intrigued when a man on the street addresses him as “Citizen.” Coming across a strange public address system, he activates and learns that they are in the Covent Garden Science Park, where a new tour is set to begin in five minutes. The Professor agrees with Ace that something is indeed wrong. A passerby informs that that the date is April 14, Accession Day. Ace comments on his bizarre lace collar.

Elsewhere in the city, Colum Devlin offers his fellow royalist Le Compte a drink. Accepting, Le Compte tells Devlin not to worry; no one saw his arrival, so they can relax and enjoy a meal. Devlin doesn’t feel like celebrating on the anniversary of the Lord Protector’s accession, but Le Compte promises that with only a small effort they will soon be rid of their great enemy. In an hour Le Compte has a meeting with one of his many agents at the Commonwealth Museum, where he will obtain vital information about the rebellion in Newcastle, where there is strong resistance to the Lord Protector’s rule. Le Compte then excuses himself to make a mysterious phone call, where he tells someone on the other end to remember what he said…

The Lord Protector’s secretary Somerset tells him of unrest and uprisings all over England’s empire, but he is more interested in tracking the comet that bears his name as it nears the Earth. She reminds Lambert that he is expected to attend the celebration of his succession, but he protests that he is sick of the needs of the people after forty years as Lord Protector; now he wants some of what the Americans call “quality time.”

The Professor learns of the current Lord Protector, Josiah Lambert, and seems to realize what’s going on. When Ace still doesn’t find the current fashion familiar, he tells her he knows a place where they should be able to get some answers. At what Ace knows as the British Museum but is here called the Commonwealth Museum, the Professor explains that they seem to have arrived in a parallel world. Noting that the technology is more advanced than in their London, he activates a computer terminal as Ace recalls that the outfit the man on the street wore reminded her of Puritan costume.

The Professor explains about the English Civil War, where Oliver Cromwell’s forced defeated and beheaded King Charles I. In the normal timeline, Cromwell was succeeded by his son Richard, a dismal failure, and the monarchy was soon restored. But that didn’t happen in this England. Here, they discover, Lord Protector John Lambert held the country together, the Protectorship became hereditary, and the Royalist cause died out during Charles II’s exile. Over time, England built up a massive worldwide empire including the American colonies, and science flourished; a massive tunnel was built under the Atlantic, and sub-orbital space flight was achieved. Ace thinks she could get used to this London, but the Professor is worried that someone has been interfering with time.

Le Compte explains to Devlin that he has urgent business elsewhere, and Devlin will have to meet his agent at the Commonwealth Museum. The female spy will be waiting on the third floor, by the statue of the fifth Lord Protector, Thomas Lambert. Le Compte departs, ordering Devlin to contact him that night. Alone, Devlin wonders why he has a bad feeling about all this.

At the museum, Ace wonders why this England is so much more technologically advanced; they even have a cure for cancer. The Professor speculates that the Republic may have laid a greater emphasis on science. But he has reached the limit of what a museum can tell him, and goes out for a newspaper, telling Ace to stay behind and improve her mind. She goes looking for the explosives homepage, but instead gets information on Thomas Lambert from a terminal near his statue.

The Professor buys a copy of the Evening Standard, paying with a diamond as he has none of this world’s currency. As he begins to read the paper, a woman in the street is struck by a car. Examining her, the Professor finds that she’s not badly injured but will need an ambulance. She murmurs, faintly but urgently, “Behemoth… Tell Le Compte that Behemoth is ready.” Wondering what she means, the Professor departs as the police arrive.

Josiah Lambert consults with his soothsayer about the meaning of the comet that bears Lambert’s name. The seer reminisces about the many times Lambert has consulted him over the forty years since their first meaning, and then reveals that Lambert’s comet will cause doom and destruction, bringing Lambert’s kingdom to its knees. Unless… “Unless?” prompts Lambert, and the seer laughs…

As the lesson on Thomas Lambert concludes, Ace is approached by Devlin, who has mistaken her for Le Compte’s agent. He claims to know who she is, and she assumes that he’s making a pass at her. Devlin’s confusion is cleared up when the Professor arrives and delivers the message about Behemoth. The news excites Devlin, who feels he must contact Le Compte immediately, so Le Compte can tell the King in person. “The King?” asks the Professor.

King Charles XIV is practicing his French and the art of skipping stones when he receives a vidlink from Le Compte, who reports that Behemoth is ready. Charles thinks that this means they will have to advance their plans. Le Compte disagrees, but refuses to explain further. He disconnects the link, leaving His Majesty to curse his agent’s vagueness. Devlin and Le Compte discuss the news. Devlin is amazed that Behemoth could be ready so quickly, but Le Compte is certain of the news; his source, a minor official at the Royal Court, has never been wrong before. Devlin asks who the man with her was, and is surprised when Le Compte claims ignorance and demands more information about the mysterious man.

Somerset is visiting the man responsible for overseeing the construction of Behemoth. She pushes aside his mention of rumors that the Protector is ailing, and admires the sight of Behemoth, the greatest rocket the world has ever seen. She can’t wait to see what happens when it’s fired at the Royalist stronghold.

At a restaurant, the Professor wonders who would benefit from altering Earth’s history. He speculates that the answer may lie in the past -- or the future -- but before he can explain further a Royalist riot breaks out in the street. Ace and the Professor flee, but Ace, shocked to see that the police are firing on the rioters, runs off to help. The man Ace rescues turns out to be Devlin, who leads her to a place where she’ll be safe from the authorities. He explains about Le Compte, and accuses her of being an agent of the Protector. She bandages his wound, using a tincture that seals the wound and covers it with plastic skin. Devlin is amazed that she has never seen such medicine before, and the two exchange more flirtatious banter. Devlin wants to stay there and relax, but Ace needs to go find the Professor. When he attempts to restrain her, Ace realizes that Devlin is holding her for questioning on Le Compte’s orders. Angry and disappointed, she leaves in search of the Professor.

Lambert is singing is his bath when Somerset comes to report that Behemoth is finished and can be fired on their enemies. He sends her away, not wanting to be interrupted in the bath. Later, he meets with his seer, who reminds him that the comet is a dangerous portent, and that Lambert must obey his orders exactly if he wants the commonwealth to survive. The seer declares that Behemoth must be fired -- but into space, not at the Royalist armies. If Behemoth is not used to destroy the comet, it will impact and destroy the planet. Elsewhere, Charles XIV learns that the Spanish and the French will join him in battle against the Protector’s forces.

The Professor has been jailed for participating in the riot, despite loud protestations that he is not a Royalist. Somerset visits him at the prison, and is shocked when he reveals knowledge of Behemoth.

Devlin catches up with Ace, and she explains how they learned about Behemoth from Le Compte’s agent. Trusting Ace’s explanation, Devlin tells her that the Professor was arrested in the riot and then taken to the palace with Somerset. She wants to go. He tells her it’s too dangerous, but relents and agrees to accompany her; he knows ways of avoiding the guards.

At the palace, Somerset threatens the Professor with execution if he doesn’t reveal what he knows of Behemoth. He answers cagily to avoid displaying his ignorance, but before Somerset can question him further, the Lord Protector intervenes, wanting to speak to Somerset in private. Left alone in her chambers, the Professor begins to snoop, and soon gains access to Somerset’s private files.

Charles XIV meets Le Compte in person for the first time. The King is prepared to cross the Channel with his armies that very night, but Le Compte disagrees. Charles protests that Behemoth will be fired soon, and they must strike quickly. Le Compte explains that he knows exactly when Behemoth will be fired, and that they can wait until the next night to invade, but refuses to explain why this is best. He departs in a hurry, preparing to see his destiny fulfilled.

The Professor hurriedly deactivates Somerset’s computer as she returns. She is upset; the Lord Protector has gone mad. After explaining Lambert’s plan to fire the rocket at the comet, she tells the Professor of The Other, the Protector’s seer/astrologer. He has been at the Protector’s side since his accession, and his family has been advising the Lamberts since the time of Cromwell. The Professor realizes that The Other’s arrival must be the point at which history diverged.

Ace and Devlin have arrived beneath Whitehall Palace via abandoned subway tunnels and sneak into the kitchens. They kiss before embarking on their dangerous mission.

Lambert explains to The Other that all is ready for the firing of Behemoth, and the mysterious man rejoices that everything is finally in place after he has waited so long. When Lambert wonders what he means, The Other explains that he is only waiting for the destruction of the Royalists, for which he has laid plans just as much as for the destruction of the comet. Relieved, the Protector departs, and The Other muses that Lambert should enjoy his reign while he can; it will soon be over, and The Other will have his revenge. Just then, Ace and Devlin stumble across him, and Devlin is shocked to recognize The Other as his fellow rebel Le Compte. The Other has them seized and taken away by guards.

The Professor is investigating the comet, and has discovered that there is no way it could hit the Earth. Indeed, when Behemoth is fired, it won’t even hit the comet at all. Somerset tells him that a young man and woman have been arrested inside the palace, and the Professor, intuiting that the woman is probably Ace, has the two brought to him. They figure out that Le Compte is The Other, and that the firing of the rocket must be stopped at all costs. Somerset doesn’t have the necessary codes, so they’ll have to break into the Lord Protector’s chambers to get what they need. The Lord Protector is reluctant, but ultimately hands the information over; it doesn’t matter anyway, as they’ll never get to Dover, where Behemoth will be launched, in time to stop it. Somerset bids him farewell, and he gives her the Protectoral Seal, telling her it may come in handy.

Somerset uses the seal to commandeer the Protector’s sub-orbital plane, and they escape as the Catholic armies arrive in London. As they get underway, the Professor explains to Ace that The Other’s target is not the comet but the astrological conjunction occurring at the same time; that would create a force of unknown power, and the Professor thinks it represents something for which The Other has been waiting for centuries.

At Dover, The Other declares that he will have revenge on the judges of Ithaca, revenge for himself and his friends. The Professor and the others arrive and begin aborting the launch as The Other reveals that he comes from a distant galaxy, far beyond even the range of the advanced technology that he has been responsible for over the past 300 years. On his homeworld he was a rebel and sentenced to death, but he escaped judgment and was launched into space, where he entered a wormhole and arrived on Earth in 1659. Ever since then, he has controlled events so that, when the wormhole is active again, he will be able to strike back at his homeworld and exact his revenge; Behemoth will obliterate the planet. His pretensions to revolution against the Protector were nothing more than a game.

The rocket launches, and a disbelieving Ace asks the Professor why he didn’t stop it. He explains that this world isn’t supposed to be here, and that the wormhole through which Behemoth travels spans time as well as space. The rocket is moving backwards through the centuries, and it will strike Ithaca before The Other even left it. The alternate timeline will be destroyed. Ace is furious with the Professor as time reverts around them.

In 1659, Cromwell is dying, and this time he appoints his son Richard as the next Lord Protector. History has been restored.