Splice

In 5087, on the war-torn alien planet of Kastarion 3, injured soldier John Francis Vater of the Anglican Marines wanders through a desolate battlefield. Blinded, he relies on the assistance of his orderly, Carson, to lead him to safety. As they walk, he contacts his young daughter, Splice, back at the Church's base. He assures her he is on his way back to her and implores her to brush her teeth, before ending the call.

The pair halt as they realise that a robotic Ambulance is close by. Carson warns John that they should avoid it, fearing it will deem him unfit for military service, but John insists they have no choice. As they continue Carson slips down a steep slope and steps on a landmine, which vaporises him. The noise alerts the robotic Ambulance, causing it to approach John. It latches onto him. Ambulance scans him, diagnosing that his blindness means his life must be terminated. John tries to resist, citing that his daughter needs him. The Ambulance insists, smelting him away.

Nearby the TARDIS has materialised and The Doctor, on hearing John's cries, races out of the TARDIS with his travelling companion Ruby Sunday following him. However, The Doctor also steps on a landmine. As the green scanner starts counting down The Doctor begins to sing ‘The Skye Boat Song’. Hearing the song, Ruby approaches him, finding The Doctor standing with one leg on the landmine and his other leg in the air. The Doctor remarks that he used the song to calm himself to prevent the detonation, believing the landmine (that he has recognised has come from Villengard a weapons manufacturing company) is scanning for signs of life to determine what has stood on it; by calming himself, he is tricking the landmine into thinking he is not there, but one wrong move and - boom!

John Francis Vater

The Doctor instructs Ruby to go find a rock, or something else heavy, so that he can counterbalance his weight when he tries to put his back down on the ground. Nearby Ruby finds a strange heavy cylinder, not realising at first that this is the compressed tissue of John. The Doctor sees that Ruby has found a possible suitable object and asks her to toss it into her other hand. This allows The Doctor to calculate if it is heavy enough for him to use to counterbalance himself. Confident that it will, he orders Ruby stay away and to toss it to him. Ruby though argues that there would be a better chance of success if she hands it to him. The Doctor at first refuses but eventually gives in, agreeing to Ruby's request to sing the song again to maintain a rhythm that will allow her pass the cylinder to him at exactly the same time he sets his other foot down.

This the succeed to do without detonating the landmine and now that he is a more stable position The Doctor takes a closer look at the cylinder that Ruby found and reads the name John Francis Vater on a plaque. Upon speaking the name, a holographic reconstruction of John appears and asks that his remains be returned to his next of kin. The Doctor and Ruby asked what happened to the man, and find out that he was terminated upon discovery of his blindness. The Doctor deduces that John's continued survival would have only harmed the budget and wellbeing of the mission, and so the algorithm dictated that he was to be terminated.

Carson

As the hologram fades away, John’s daughter Splice arrives at the crater looking for her father after having received the message from her father earlier and tracking it back to its source. The Doctor and Ruby look around nervously and introduce themselves to her. When Splice introduces herself John Francis Vater's hologram whirs back to life and starts talking to her, apologising for her loss. Splice though does not understand and on seeing what she thinks is her father she starts to run towards him and so towards The Doctor and the landmine. Ruby is forced to keep Splice away so she won't trigger the landmine. The situation though has caused the timer on the landmine to countdown further as The Doctor struggles to keep himself calm.

Then an Anglican Marine arrives and from above the crater insists that The Doctor release the casket that he is holding, as it contains the remains of an ordained Anglican. With her gun still trained on The Doctor the marine, Mundy Flynn, explains that they are fighting Kastarion aliens that are thought to be living underground. She says that there is no reason for him not to release the remains of John Francis Vater to her as The Doctor is dead already. She explains that even if the landmine does not detect a viable target it will still explode after a certain time as a failsafe. So as to keep calm The Doctor banters back at her. Mundy though is not amused by this and determined to make him drop the casket she shoots The Doctor’s arm but The Doctor holds on regardless, enduring the pain as the timer on the landmine countdowns further again.

The Doctor asks Mundy how the landmine actually works, since he recognises that it does not have an explosive component. Mundy explains that the landmine is triggered by affecting the DNA of whoever steps on it and turning them into the explosive. The Doctor reveals that as he a Time Lord this means his biology will create an explosion far bigger than she expects. She scans him and determines that he is right; he could wipe out half the planet. However, Mundy's attack has injured The Doctor, prompting an Ambulance to enter the crater and scan The Doctor. Panicked, Ruby takes Mundy's rifle and fires it into the air to draw the Ambulance away from The Doctor so that it doesn't treat him. But Mundy reveals that this will only work if one of them is injured as well. Revealing that her gun is on its weakest setting, she hands it over to Ruby and orders her to shoot her arm. As Ruby hesitantly aims the gun at Mundy another Anglican Marine called Canterbury James Olliphant (also known as Canto), who harbours a crush on Mundy, arrives and, believing Ruby is about to kill Mundy, shoots Ruby, leaving her severely injured.

Activating a Mine

This though does result in the Ambulance switching its focus from The Doctor to Ruby. Ambulance identifies Ruby’s age but cannot locate her next of kin, stumbling again and again. As it tries snow begins to fall. Then the snow stops, freezing in mid-air. The Ambulance determines that Ruby has approximately 432 seconds to live, but as she's not ordained, withholds treatment. Mundy and Canto try to save Ruby, to keep her alive, but the Ambulance is programmed to not heal unbelievers.

The Doctor, having deduced earlier, reveals the truth: there are no Kastarions and Villengard is making money simply off the soldiers being there. To save Ruby, to stop the landmine from going off and the Ambulances killing anyone else the Anglian Marines need to surrender. He explains that when the marines arrived six months ago and set up their defensive perimeters, they launched the Ambulances' acceptable casualty algorithm. Doing this they set in motion a cycle of attrition and war against their own hardware, grinding themselves down just enough to keep themselves engaged, keeping the combat ongoing, and keeping the money flowing to Villengard. Mundy though is sceptical and demands proof, and so The Doctor calls for John Francis Vater's hologram, ordering him to enter into the Ambulance's computer to find proof that the planet is uninhabited and the deaths are entirely self-inflicted. John Francis Vater insists that this goes against his protocols, but The Doctor insists he is still Splice's father and has a duty to help her. John finally relents, reminds his daughter to brush her teeth, and disappears into the Ambulance.

While trying to reconfigure the Ambulance treating Ruby, Canto is killed. In his cylinder, Canto admits to Mundy how much he loves her. Other Ambulances then arrive surrounding the crater, the Algorithm having sent them to prevent a data breach, claiming it had destroyed John Francis Vater's hologram. The landmine beneath The Doctor's feet begins its failsafe detonation process, meaning the planet will explode. Then all the Ambulance's begin to glitch as it is revealed that John Francis Vater's hologram has managed to override their protocols, resulting the lead Ambulance to heal Ruby. The landmine also turns green, meaning The Doctor can safely step off it.

A short time later, The Doctor explains that John's love for his daughter defeated the Ambulances. As the group admire the sky the marines prepare to leave the planet, the false conflict now over. However, The Doctor soon gets bored and insists he and Ruby depart. As they return to the TARDIS, The Doctor and Ruby look back towards Splice and Mundy who are admiring the sky (Mundy having previously promised John that she would look after Splice). Alongside them is John Francis Vater's hologram who waves The Doctor and Ruby goodbye.