The Battle of Floor 0507 was a skirmish between the Twelfth Doctor and the residents of Floor 0507 against the Cybermen invading from Floor 1056 on a Mondasian colony ship. Initially ruled by the Master, the Cybermen turned on him when the Doctor rewrote their definition of humanity, leading to the group fleeing to the upper floor while the Cybermen began preparations for the final stage of Operation Exodus: the conquest of the entire ship.
The Cybermen held a significant advantage as they were further away from a black hole, meaning years for them was mere days for the Doctor and the others on the solar farm, allowing the Cybermen to spend hundreds of years preparing. (TV: The Doctor Falls)
After the Master and Missy prevented a patrol of CyberNeomorphs from weakening the humans with a swarm of Cybermats, (PROSE: Alit in Underland) and a battle between the humans and hundreds of highly evolved Cybermen, the skirmish was eventually ended when the Doctor, having had Nardole evacuate the majority of the residents to Floor 0502, used his sonic screwdriver to ignite the floor, destroying all the Cybermen present at the cost of forcing him to regenerate. (TV: The Doctor Falls)
Genesis of the Cybermen
Due to the colony ship being sucked into the orbit of a black hole, 20 members of the 50 member crew on board went from Floor 0000 to Floor 1056 to reverse the rear thrusters. However, the time dilation meant they were unable to return to Floor 0000, and so an entire civilisation was formed over the centuries that passed before the residents could return to the top of the ship. Unfortunately, as Floor 1056 was never designed to house hundreds of people in such close proximity to the ship's engines, this meant that the city gradually began to regress and, after a thousand years of occupation, the city had become an industrial wasteland, rife with poverty and sickness. (TV: World Enough and Time)
At some point, the Master, having escaped Gallifrey after causing Rassilon to regenerate (TV: The End of Time) and having his "little condition" cured by the Time Lords, crashed his TARDIS on Floor 1056 and took control of the city for a time, eventually steering the residents down a path to convert themselves into cybernetic humanoids, and eventually complete Cybermen. The "patients", some of the earliest Cybermen, would venture to the higher floors and capture the other humans for conversion. A group went to Floor 0507, but were repelled by the residents, being shot down daily. (TV: The Doctor Falls)
Eventually, the Twelfth Doctor, Bill, Nardole and Missy landed on the ship, finding the sole survivor on Floor 0000, Jorj, who was spared from capture due to not being completely human. When a number of lifts began rising to their floor from Floor 1056, he shot Bill due to her being a human; when the lifts arrived, the patients took her away. Bill was later kept under the Master's watch (who was masquerading as "Razor" at the time) at the hospital until he had her converted into the first complete Mondasian Cyberman. The Doctor, Nardole and Missy followed after knocking Jorj out and figuring out the time dilation between the numerous floors. However, the Doctor was too late to save Bill, and Missy, after discovering the ship's planet of origin - Mondas - and then finding her predecessor, joined the Master, siding against the Doctor as they gloated about the "genesis of the Cybermen". (TV: World Enough and Time)
The pair then captured the Doctor, knocking him out and taking him to the top of the hospital where they gloated about their victory. However, the Doctor revealed that he had changed the definition of humanity in the Cyber network, turning the army against the Master. After fending the initial group off with his laser screwdriver, the Master was knocked out by Missy using her sonic umbrella, who then freed the Doctor from his restraints. Nardole then arrived in a shuttlecraft, the group proceeding to use it to evacuate. However, when the Doctor was electrocuted by a Cyberman, Bill saved him using her cyber-weapon. She then forced the two Masters to take her and the Doctor with them. The shuttlecraft eventually crashed on Floor 0507. (TV: The Doctor Falls)
The night after the shuttlecraft crashed on Floor 0507, the Master and Missy attempted to travel to Floor 0508 via the walkways between the two floors; although the Master had claimed to Nardole and Hazran, the leader of the humans on Floor 0507, that he was going to find a way of stopping the "scarecrows" and preventing "the exodus", he actually intended to find a way of reversing the Doctor's meddling to the Cybermen on Floor 1056 and regain control of the Cyber-Army. However, the two Masters were eventually discovered, and followed, by Alit, a young girl who lived under Hazran's care, after she was pursued by a patient attacking the solar farm; said patient pursued the group until the Master successfully reprogrammed it to follow his commands, before being renamed "Topknot" by Missy. The group then found a service lift and arrived on Floor 0508.
Spending most of the next day riding various forms of autonomous farming equipment to find a means of accessing the Cyber-network, the Master, Missy, Alit and Topknot eventually discovered a large number of storage silos containing farming equipment, only to be discovered by a patrol of CyberNeomorphs. The patrol's Cyber-Leader explained that they had been sent from Floor 1056 by the Cyber-Planner to capture the two Masters for interrogation as to how the Cybermen's definition of humanity had been altered. After Missy briefly stunned the Cybermen with her sonic umbrella, the Master destroyed the Cyber-Lieutenant with his laser screwdriver before being knocked out and held prisoner by the Cyber-Leader. With Topknot remaining hidden from view, Missy and Alit escaped from a pair of Cybermen whilst overhearing the Cyber-Leader ordering another pair of Cybermen to prepare a swarm of Cybermats for an attack on Floor 0507.
Having thrown off the Cybermen's life-sign scanners by capturing a number of rats and caging them around her and Alit's necks so as to register as life-forms with three hearts, Missy and Alit escaped into another silo. Here, Missy hacked into the ship's systems, discovering, and reactivating, an old failsafe system in the Cybermats originally designed to keep the patients inside the hospital on Floor 1056, thereby sending the entire swarm back to the bottom of the ship and deadlocking the system so that the Cybermen couldn't immediately reactivate it. Missy then forced the weather outside to rain before escaping with Alit and Topknot. Recovering the Master, the trio attempted to use Topknot as a distraction for the Cybermen only to be ambushed upon trying to escape. However, Missy then instructed her predecessor to fire his laser screwdriver at the steel ground, generating a huge electrical burst that destroyed the remaining Cybermen; the trio survived thanks to the insulating abilities of the boots they were wearing after finding them in a storage silo. The group then rode another service lift to a stable on Floor 0507, before riding on a horse and cart back to the solar farm. Alit noted that Missy could have stopped all of the Cybermen rather than just the Cybermats, but Missy told her to keep quiet about it. Missy then concluded that killing her predecessor was the only way to stop him and his plans. (PROSE: Alit in Underland)
Preparing for war
Over the next two weeks, Nardole took command of the village, having the residents prepare for the upcoming attack from the Cybermen, fortifying the town with sandbags. Missy and the Master took to locating the lifts on the level, eventually finding and revealing one to the Doctor and Bill. However, Missy accidentally called the lift to their level, forcing the four to fight off a highly advanced Cyberman, as centuries had passed on Floor 1056.
Realising the Cybermen now knew where they were and would now have years to prepare an invasion while the humans would have only a day, the Doctor returned to the village to rally the residents for the battle, the Cybermen sending a warning in the form of an alarm in an effort to frighten them. Understanding they would need a way out, the Doctor had Bill blast a hold in the wall to an underground passage leading to the lifts while the Masters schemed to flee to the Master's TARDIS on Floor 1056.
Nardole used a computer to hack into the fuel lines beneath the floor, enabling him to detonate them on command. The Doctor decided they would use such a tactic to make the Cybermen believe they had superior weapons so they would not attack in the first wave, at least for a while.
Shortly before the Cybermen arrived, the two Masters revealed their plan to leave to the Doctor. Infuriated, the Doctor pursued them, proceeding to criticise the pair for their indifference and that he was staying to fight because it was "kind" and "right". Although the Master refused to listen and left, Missy was moved by the Doctor's speech, considering siding with him. However, she sided with the Master, but thanked the Doctor for trying to make her good. (TV: The Doctor Falls)
The initial wave of Cybermen arrived, marching towards the farmhouse. However, they were met by Alit, who tossed an apple at them. Nardole, using his computer, ignited the fuel line beneath the platoon, blowing them up. The Master and Missy manoeuvred through the forest to the lifts, witnessing the explosions in the distance and avoiding the Cybermen. Nardole proceeded to destroy several of the Cybermen, protecting the humans, and prompting the Cybermen to retreat. Understanding they had little time before the Cybermen returned with their entire army, the Doctor downloaded Nardole's software into his sonic screwdriver and, after a long argument, convinced Nardole to go with the evacuating children to protect them. Bill also opted to stay with the Doctor, wishing to die defending the children rather than continue living as a Cyberman.
In the forest, the Master and Missy successfully reached the lift. However, having changed her mind, Missy tricked her predecessor and stabbed him in the back, asserting she wanted to stand with the Doctor. This wound would force the Master's regeneration into Missy, though she made it precise so that he would have enough time to return to his TARDIS first. Helping the dying Master into the lift, she prepared to return to the main site of the battle. However, the Master struck her down with his laser screwdriver, professing he would refuse to let her help the Doctor. Laughing at their "perfect ending", the Master descended in the lift to his TARDIS on Floor 1056, where he regenerated into Missy and left the colony ship. However, despite Missy's apparent permanent death, (TV: The Doctor Falls) she was able to begin a new regeneration cycle. Cryogenetic extraction destroyed her body but kept a copy of her consciousness prior to regenerating into her new body, renaming herself the Lumiat. (AUDIO: The Lumiat)
By morning, Bill and the Doctor knew the rest of the Cyber-Army were imminent, sharing one last goodbye with Bill before she left. Alone, the Doctor reiterated the words his wife River Song once told him: "without hope, without witness, without reward." The second wave arrived, the Doctor and Bill each charging into opposite ends of the forest, fighting off various types of Cybermen. As the Doctor destroyed numerous models of them, he shouted out the numerous occasions he had beaten them. Eventually, he was surrounded and struck down by one Cyberman's laser beam. Wearily declaring he was still the Doctor, the Cyberman rebuked him and fired again. As his body attempted to regenerate, the Doctor ignited the entire floor, destroying all of the Cybermen and mortally wounding himself in the process. (TV: The Doctor Falls)
Safe on Floor 0502, Nardole stood outside the lifts, wondering if the Doctor and Bill would return, though Alit eventually convinced him to move on and focus on living with her and the other children. Nardole stated to Alit that the Doctor's actions had only destroyed "most" of the Cybermen, but had bought them some time for Nardole to come up with a new plan.
Below, on Floor 0507, the Doctor had become motionless after the explosion which had destroyed all the Cybermen but Bill. Bill was then confronted by the oil creature in the form of Heather, which had tracked Bill down due to leaving itself as residue in Bill's tears. It turned her into a creature like itself, and together they took the Doctor back to the TARDIS. Heather invited Bill along to explore the universe with her and, after bidding the Doctor farewell, Bill and Heather left.
After Bill and Heather's departure, the Doctor awakened and began to regenerate. (TV: The Doctor Falls) He eventually finished the process, regenerating into the Thirteenth Doctor. (TV: Twice Upon a Time)
The Cybermen launched further attacks but they were continually fought off by Nardole. (WC: The Best of Days) The threat was reduced to minor Cybermat incursions taking place every spring by the time Nardole died at the age of 728. (PROSE: Twice Upon a Time)
Behind the scenes
Although TV: The Doctor Falls does not mention the Cyber-Planner of PROSE: Alit in Underland by name or even refer to a potential leader of the Cybermen beyond the Master, it is likely that said Cyber-Planner also leads the Cybermen during their attack on Floor 0507 in TV: The Doctor Falls. Similarly, although it is never mentioned by name, it is also likely that this Cyber-Planner is the one responsible for the continuous Cybermat attacks on Floor 0502 that are mentioned by Nardole in PROSE: Twice Upon a Time.