Hours after the tragic events at the Forge, Evelyn finally calms down enough to emerge from her room in the TARDIS; however, she’s still deeply upset by the Doctor’s apparently casual reaction to Cassie’s death, and is no longer sure she wants to travel with him. The Doctor, desperate to make things right, suggests that she take some time out on the Eye of Orion, but she does not want to be entirely alone; she wants to be amongst ordinary people she can talk to without having to face a threat to the world or the Universe. The Doctor’s somewhat pedantic and flippant response only upsets her more, and, realising this, he backtracks and promises to find somewhere suitable.

On the planet Világ, the countries of Galen and Malendia have finally negotiated a ceasefire after centuries of warfare. Their new alliance is to be cemented by the marriage of Galen’s Princess Krisztina to Malendia’s Prince Viktor, and even though neither truly loves the other, they have agreed to go through with this for the sake of their countries. Cheering crowds have gathered outside the palace in the capital of Galen, and Governor Rossiter of Kozepén, a small country located between Galen and Malendia, is preparing to address them. But at least one person isn’t happy: Plenipotentiary Suskind of Malendia, who opposed the peace process from the start. He has had little choice but to bow to the will of his cabinet and attend the celebrations alongside his opposite number, Paramount Minister Mortund of Galen; however, Suskind believes that his country is making a terrible mistake in forming an alliance with their traditional enemies, and has put plans in motion to ensure that this fragile alliance does not last.

Princess Krisztina is to make a public appearance as well, but she requires a bodyguard, as there are still those who oppose the Alliance and call Krisztina a traitor. Krisztina herself does not know whether she’s doing the right thing, and on the way to the ceremony, she asks her bodyguard for his personal opinion: is she doing the right thing in agreeing to a loveless marriage for the sake of her country? His reply is a bit too personal, however, and Krisztina, noting that he is not wearing a name badge, demands that he identify himself. He reluctantly removes his mask to reveal that he is Marcus Reid, a man whom Krisztina once knew very well...

Meanwhile, Rossiter addresses the cheering crowds. As independent arbiter of the peace talks, it is his duty to announce the formation of the Kingdom Alliance, and the end to centuries of war. But as he speaks, the plaza is rocked by an explosion, and protestors storm the palace, apparently confirming Suskind’s fears that the ordinary people do not support the Alliance after all.

The Doctor has chosen to take Evelyn to Világ, and he explains the political situation to her as she packs. The attack on the palace was the last major violent incident before the formation of the Kingdom Alliance; the planet is due to be invaded by the vicious Killorans in a few weeks’ time, but the Alliance’s combined might will see them off, solidly cementing the two countries’ relationship, and there’s no need for the Doctor or Evelyn to stick around for any of that. The TARDIS materialises in the palace’s botanical gardens, but as the Doctor shows Evelyn out, she storms off alone, still upset with the Doctor -- and with herself for reacting so angrily. She’s not admitted to the Doctor that she has a heart condition, and she’s walked too fast in the heat -- and as the beautiful chiming of the flowers echoes in her head, she keels over on the path...

As the Doctor had promised, it’s been over a week since the protestors’ attack. Governor Rossiter has returned to speak with Krisztina, who is out in the gardens, presumably checking on repairs to the palace grounds. Krisztina is tending to Evelyn, who fainted right in front of her but is now recovering; the chiming xenoblossoms may have overwhelmed her. Evelyn and Krisztina soon hit it off, and when Rossiter arrives, Krisztina asks him to escort Evelyn to the palace so she can rest and recover from her faint. Before leaving, Rossiter speaks privately with Krisztina, cautioning her not to do anything foolish; his daughter, Sofia, has told him of what passed between Krisztina and Marcus five years ago. Upset, Krisztina informs him that she knows where her responsibilities lie, and that she plans to tell Marcus that they can never see each other again.

The Doctor has been quite hurt by Evelyn’s anger, and he tries to forget his troubles by going fishing; however, he is interrupted by a young man who is skipping stones from a nearby bridge. The Doctor confronts the young man, Lieutenant Marcus Reid, and eventually ends up skipping stones along with him while Marcus pours out his troubles. Ten days ago, Marcus was reunited with the love of his life, and they each helped rescue the other during the attack on the palace; Krisztina managed to pry open the lift’s inspection hatch, and Marcus shot two protestors who’d forced their way into the building and were about to attack Krisztina. When Governor Rossiter arrived, Marcus has just enough time to ask Krisztina to meet him here, at the bridge where they used to spend their evenings; he’s been waiting here every night for the past ten days.

Dissatisfied with the “terrorist” attack on the Galen palace, Suskind contacts Commander Pokol, the head of a black ops squad that has been arranging violent anti-Alliance demonstrations in Malendia. Officially, Pokol and his men are to act as Suskind’s bodyguards; unofficially, they are to stir up trouble in Galen and ensure the end of the Alliance by whatever means necessary. Meanwhile, Rossiter and Evelyn chat about politics and history on their way into the palace, and Rossiter is impressed by Evelyn’s forthrightness, intelligence, insight and idealism; however, he is too embarrassed to tell her his first name.

Krisztina is now torn between her duty to her country and her love for Marcus, and the Doctor, who realises that this situation won’t easily be resolved, urges them to continue the conversation in private. Marcus and Krisztina thus lead him to the riverbed, where a loosened grille gives them access to the abandoned baths. There, the Doctor tries and fails to undo the damage he’s caused. Five years ago, Marcus ran away and joined the army, convinced that a lowly gardener’s son would never be able to marry the princess of an entire country and that he and Krisztina were only torturing themselves pretending otherwise. But he’s thought about Krisztina every day since, and now, thanks to the Doctor’s inspiring advice, he’s found the courage to admit his true feelings. Krisztina in turn admits that she still loves him, even though he ran out on her five years ago without a word. The Doctor gives in to the inevitable and offers to help them; the marriage must go ahead, and the Kingdom Alliance must be formed, but the Doctor will arrange for Krisztina and Marcus to continue seeing each other in secret.

Over the next few weeks, there are sporadic outbreaks of violence in both countries, but nothing that seriously threatens the Alliance. The Galen ambassador to Malendia is killed in a drive-by shooting, but Mortund himself takes his place. Suskind remains in Galen, but refuses to comment on the violence. Krisztina appoints Evelyn as special envoy to Kozepén, where she and Rossiter oversee the consolidation talks. The media is all aflutter over the Princess’ new appointment, a mysterious advisor known as the Doctor who claims to be her astronomy tutor. Four weeks after the attack on the palace, Marcus Reid is to be presented with an award for bravery and appointed to the Princess’ personal bodyguard...

Meanwhile, Suskind and Pokol are keeping a close eye on the Doctor. Pokol just wants to kill him and have done with it, but Suskind is unwilling to risk such a prominent murder being traced to him -- and in any case, since Krisztina is too well-protected to attack directly, Suskind intends to get at her through her friends...

The Doctor has been trying to contact Evelyn, but she’s not returning his phone calla; however, he knows that this is something they must deal with themselves, and turns down Krisztina’s offer to speak with Evelyn on his behalf. Over the past few weeks, he’s been passing love letters between Marcus and Krisztina, hidden in her “astronomy homework,” and after today’s ceremony, the lovers will be able to see each other every day. Krisztina and the Doctor set off for the ceremony, and on the way, Krisztina compliments the Doctor on his new buttonhole, a rose provided by a friendly staff member from the palace gardens. Oddly, when they arrive at the ceremony, Suskind -- who is there under protest -- also compliments the Doctor on his buttonhole...

The Doctor sits between Krisztina and Marcus in order to keep their public flirting to a minimum, but is more disturbed when Pokol arrives and whispers something to Suskind, who seems disturbingly cheered by the news. The Doctor follows Pokol as the ceremony begins, and finds him ordering his men to search Krisztina’s astronomy work for the love letters she’s written to Marcus. The rose in the Doctor’s buttonhole is a surveillance device. The Doctor, desperate to stop Pokol, attacks him and seizes his stun-baton, but Pokol is a trained soldier and does not hesitate to attack the Doctor in turn and retrieve his weapon. He zaps the Doctor unconscious and sets about his work -- exposing the love affair between Krisztina and Marcus and putting an end to the Kingdom Alliance.

Over the next two weeks, everything falls apart. Krisztina and Marcus’ affair is made public, Suskind withdraws the offer of marriage on Viktor’s behalf, and Galen and Malendia find themselves at war once again. Within two weeks, thousands have been killed, and Marcus is assigned to the front lines of the war. The Doctor is placed under house arrest, and then confined to prison when he tries to escape from the palace. A failed assassination attempt puts Mortund in hospital in Malendia. And extradition proceedings are brought against Evelyn, who will have to leave Kozepén and face charges of conspiracy.

Rossiter has done all he can for Evelyn, but she still insists that the one thing he must do is take a side in the war. She is unable to explain why it’s so important that he form a coalition, however, and he can only assume that she wants him to side with one country to defeat the other, which makes no sense. He’s concerned for her health and even for her sanity, but can do nothing to prevent her extradition. She will be transported by train, since airspace over Galen has been closed to commercial flights. The man from immigration and security searches Evelyn’s handbag for dangerous items and marches her off, rushing her past the reporters outside without giving her a chance to speak. Once aboard the train, he leaves her in her compartment while he gets something to eat, but he fails to realise that she can use the cold cream in her handbag to slip out of her handcuffs. Still hoping to convince Rossiter to form a coalition, and knowing that her heart condition will kill her eventually even if the alien invasion doesn’t, she leaps from the train.

Suskind has remained in Galen’s capital, ostensibly for diplomatic purposes, and when he hears that the Doctor has been trying to contact Mortund, he visits the prison to hear what the Doctor has to say. The Doctor’s cellmate, a young man beaten and thrown into jail for participating in a peaceful pro-Alliance demonstration, sleeps through the Doctor’s attempts to convince Suskind that he’s made a terrible mistake; united, nothing could stand against Galen and Malendia, but divided, their armies will beat each other into submission. Suskind follows the Doctor’s argument through to its logical conclusion, but dismisses it; the only possible common enemy the two countries might face would be a coalition of the smaller republics led by Rossiter, and Suskind knows him well enough to dismiss this as a possible threat. But the only other possible threat would come from off-world, and though it’s an interesting possibility, it’s not one Suskind believes will ever come to pass.

Suskind leaves the frustrated Doctor locked up and fuming in his cell, but finds he can’t quite dismiss the implications of their conversation. He thus contacts Mortund in Malendia, and asks him to discuss the possible benefits of the Kingdom Alliance had it gone ahead. Though it takes some prompting, Mortund reaches the same conclusion by himself: it’s possible that the combined military forces of the Alliance could have sent any potential invaders packing, but the current political situation may have left both Galen and Malendia open to attack. Neither he nor Suskind know of any such invaders, but the conversation nevertheless leaves Suskind disturbed.

Krisztina decides that she’s had enough, and thus has her chauffeur take her to the prison; there, she forces her way past the guard at the front door, knocks out a second guard with a dinner tray, and frees the Doctor from his cell. The Doctor intends to contact Mortund and talk some sense into him, but Pokol -- who’s been keeping Krisztina under surveillance -- arrives and holds them both at gunpoint. He’s killed both guards, and now he shoots the Doctor’s young cellmate as he sleeps; he intends to shoot Krisztina as well, make it appear as though the Doctor shot her in a botched escape attempt, and kill the Doctor in “self-defence.” Krisztina rushes Pokol, who shoots her in the arm, wounding her. The Doctor manages to get the gun away from Pokol, but Pokol knows the Doctor will never use it; however, when Krisztina grabs the gun herself and threatens to kill him, he can tell she’s not bluffing. Krisztina locks him in the cell before collapsing in the Doctor’s arms; fortunately, she’s only suffered a flesh wound, and she helps the Doctor to escape by pretending that he’s taken her hostage.

Evelyn awakens to find herself in a hospital in Kozepén with Rossiter by her bedside. She managed to knock herself unconscious and dislocate her shoulder by jumping from the train, but she’s also managed to convince Rossiter that she must have deep and fundamental reasons for wanting him to go to war. However, he still needs actual evidence in order to convince the rest of his government. Sadly, Evelyn fears that that evidence is on its way...

As the bewildered Krisztina tries to understand what the Doctor is telling her, the Doctor tries to work out the Killorans’ likely trajectory through the atmosphere, and concludes that they’ll hit Galen before Kozepén. Assuming that Evelyn has been deported on schedule, he decides that the first priority must be to get Krisztina to safety in the palace and then speak to Suskind; he and Krisztina are still wanted fugitives, but soon the security forces will have a much bigger problem on their hands...

In Kozepén, Evelyn finally admits to Rossiter that she has a heart condition, only to learn that he already knew this; she’s been hooked up to a monitor since she arrived in hospital. She can now admit that she began travelling with the Doctor because the excitement helped her to forget about her condition, but the deaths she’s seen recently have made her consider her own mortality once more. She now realises that this is why the Doctor’s apparent ability to shrug off Cassie’s death has hit her so hard; will he forget her just as easily when she’s gone? Rossiter comforts her as she cries, and helps her to understand that the Doctor must still feel grief, even if he doesn’t show it the way she does. Military officers burst in to inform Rossiter of the state of emergency, and Rossiter insists that Evelyn accompany him as they board a command plane.

On the front lines of the Galen/Malendia war, the soldiers on both sides suddenly find themselves fighting an entirely different battle as the Killorans descend. Word of the invasion soon reaches the Galen capital, where it soon becomes clear that both sides are taking a beating -- and that the Malendian military is stretched too thinly to defend their country. Suskind orders Pokol to get him to the Galen palace, but they arrive just as the Doctor and Krisztina do -- and Pokol, still enraged and humiliated by the Doctor’s escape, draws his gun and fires indiscriminately into the panic-stricken crowd. Before he can hit the Doctor, the Killorans arrive, and Pokol is struck down as they strafe the palace.

Marcus manages to organise some of the surviving soldiers, and since they’re outnumbered on the front lines, they retreat to Galen’s capital to regroup. Meanwhile, Rossiter tries to organise his own military to defend Kozepén, but soon realises that this alien invasion is the threat that Evelyn’s been trying to warn him about for weeks. Though unable to explain how she knows what she does, she convinces Rossiter that, instead of consolidating to defend his own country, he must send his troops into the fray, joining forces with Galen and Malendia to take the fight to the invaders. Only then will the Killorans be driven away from Világ.

The Doctor and Krisztina get into the palace, and while the Doctor tries to find a phone and speak with Rossiter, Suskind admits to Krisztina that he’s responsible for many of the anti-Alliance demonstrations, including the attack on the palace, the assassination of the Malendian ambassador and the attempt on Mortund’s life. He thought his country stood stronger alone, but now he’s seen how terribly wrong he was, and once the true enemy is defeated, he will take responsibility for his crimes. The Killorans then storm the palace, and Suskind and Krisztina stand together against the invaders; however, a Killoran scout tracks down the Doctor and throttles him.

Marcus and his men arrive back in the capital to see the palace under attack, and Marcus’ first thoughts are for Krisztina. He and his men storm the palace, but in the ensuing exchange of gunfire, Marcus is struck down. Badly injured, Marcus manages to hold himself together long enough to organise his men, who keep the area secure until reinforcements arrive. United, the armies of Galen, Malendia and Kozepén successfully drive off the Killorans.

Rossiter and Evelyn return to the capital to find the palace in ruins, but Rossiter assures Evelyn that it will be remembered and rebuilt. Recalling the last time they entered the palace together, he finally admits that his first name is Justice; his mother always wanted a boy, and that’s what she cried out when he was born. This tender moment is broken when they enter the palace to find Krisztina weeping over the body of Marcus, who has finally succumbed to his injuries. Suskind then arrives with the Doctor, who used his respiratory bypass system to convince his attacker that he’d been asphyxiated -- but as the horrified Doctor takes in that Marcus has been killed, Pokol bursts into the palace, sees the Doctor, and starts shooting. Pokol ignores Suskind’s order to stand down, and Suskind shoots and kills him personally, but he is too late; Pokol has shot and killed Krisztina. The Doctor screams in rage and despair and rushes off, and Evelyn, realising that her friend is about to make a terrible mistake, bids a hasty but fond farewell to Rossiter and rushes off after him. Rossiter is sad to see her go, but knows not to interfere.

Evelyn barely makes it back to the TARDIS before the Doctor takes off. Blaming himself for the deaths of Marcus and Krisztina, he intends to return to the river and intercept Marcus before he reaches the bridge, ensuring that this tragic chain of events never takes place. Evelyn manages to convince the Doctor to freshen up, rather than introduce himself to Marcus while wearing a coat stained with blood -- but as soon as the coast is clear, she re-sets the time co-ordinates. The TARDIS thus materialises five years too early, and when the Doctor reaches the bridge, he sees a much younger Marcus and Krisztina giggling and talking. By now, he’s calmed down somewhat, and Evelyn stops him from returning to the TARDIS, telling him to look at the two young lovers and see them for what they are. If not for the Doctor, they would have lived out the rest of their lives in denial, and died alone; the Doctor gave them a chance to die in each other’s company.