Meeting the Master
Cole was piloting a ship coming down to Arcking, when the Daleks, in search of the War Master, attacked the ship, killing most people on board apart from him and a small number of others. He was placed in medical care, where he met the Master, going under the alias "Doctor Keller". Cole, badly affected by survivor's guilt in the aftermath of the incident, told the Master that he wished he could be more proactive in saving some of the many people affected by the Time War. The Master commended him for this attitude and suggested that he would like to discuss the subject further with him at some point.
When the Master asked for a pilot so he could use a landship to search for an energy source, which was rumoured to be why Arcking had such mysterious properties, he requested Cole as a pilot, alongside Phila as a companion, albeit briefly. Upon finding the source, they found that it was sentient. After leaving it, due to it talking to them and refusing to go, their landship was attacked by Daleks, and they were forced to leave it. Phila was killed, and Cole left Arcking with the Master in his TARDIS. (AUDIO: The Good Master)
Companion to the Master
Cole eventually grew weary and annoyed with the Master's decision to focus on the bigger picture of the Time War, and argued that he should focus more on helping some of the innocent people who were in peril from its effects. The Master replied that while he, as a Time Lord, was not allowed to directly interfere with local affairs, he would allow Cole a chance at playing the role of a saviour. He then showed Cole a chart over all nearby planets in danger from the War, and asked him to pick one to save. Although reluctant, Cole picked a planet, and he and the Master went to explore it.
Cole and the Master discovered that the planet was a somewhat primitive planet, populated by peaceful farmers. Cole noticed that there didn't seem to be any immediate threat, and the Master agreed, realising that their stay on the planet was maybe going to be a prolonged one. Reminding Cole that he could not interfere, the Master left Cole at a village, while he went to the neighbouring town, wanting to learn go grow grapes and establish his own vineyard. Cole was met with curiosity by the villages, who were somewhat apprehensive about the "man the from sky", but when Cole declared that he came in peace and had landed on their planet with the intent to save them, they reacted with amusement and they accepted him into their community and put him to work. For next weeks, Cole went around the village and used his skills as an engineer to make improvements to their primitive technology, and he gradually got romantically involved with a local woman named Elidh and eventually also earned the trust of Elidh's father, Anvar. During this time, Cole also paid occasional visits to the Master, who reacted with bemusement to his stories of his involvement with the local community. One evening though, as he and Elidh were stargazing, several objects fell down from the sky and landed around the planet. Cole realised that the threat the Time War posed to the planet's people had arrived.
After that evening, strange things started happening around the planet; the farmers and their livestock started getting sick and their crops were dying. By seeking advice from the Master, Cole figured out that the objects that had struck the planet had probably landed there by pure accident, and were now leaking Temporal Decay all over the planet, this being the cause of all living things getting increasingly weaker. Cole also discovered that he and the Master were completely unaffected by the effects of the Temporal Decay, believing that it had something to do with them being time travellers. As Cole looked for a solution to the problem, he also had to deal with the local community growing increasingly suspicious and hostile towards him as they gradually started to starve and become sick from the fallout, despite Elidh and Anvar trying to assure them that he was trustworthy.
Cole eventually came upon a solution to the problem as he conceptualised a radiation suit, based on his own space suit, which would shield the people of the planet against the decay and thereby keep them alive, although they would have to wear the suit at all times. He explained his idea to the Master, who, although still refusing to get directly involved, "accidentally" dropped off some supplies and tools to help out Cole in his endeavour. Cole tested the first prototype of the suit on Anvar, as he had been seriously weakened by the effects of the fallout, but when he discovered that Anvar, who had turned delirious from his sickness, panicked upon being placed in the suit, he decided to install emotional dampeners in the suit to keep the user conformable and preventing them from trying to take it off. For his second test he used Elidh as a test subject, as she was the one he was most desperate to save. Elidh, however, discovered that Anvar had died from sickness as Cole had been more focused on saving her, and broke off their romance.
Undeterred, Cole continued to make more suits and managed to convince all of the villagers wear one, and set up a device capable of replicating the suits, before tasking the villagers, who treated him with gratitude, to find any other survivors on the planet and keeping them safe by placing them into the suits too. Eventually, however, Cole discovered to his horror that the suits gradually turned them into robot-like creatures, obsessed with seeking revenge on the aliens who had poisoned their planet. Eventually the robotic villagers turned on him too, as they came to see him as an alien, chasing him across the countryside. Cole was eventually hunted down by one of his pursuers, but by chance it happened the robotised Elidh, who could dimly recognize him, and still remembering their past romance, she allowed him to escape. Cole rendezvoused with the Master, after which they left the planet.
In the TARDIS, the Master, using his Time Lord knowledge, informed Cole of the full consequences of his meddling, namely that he had created an entirely new species which was not supposed to exist in the first place. He told Cole that the fact that the species was warlike and obsessed with seeking out revenge on some other species they did not even know the true identity of would motivate them to journey into space, conquering numerous planets and annihilating many innocent lifeforms, and in the process they would do untold damage to the already weakened Web of Time. Horrified by what he had done and the implications of what the Master had told him, Cole desperately asked the Master to help him undo his mistake, telling him that he was willing to do anything he was asked of in exchange. The Master took it in stride, cryptically telling Cole not to worry, as he assured him that he would take care of it. (AUDIO: The Sky Man)
The Master took Cole to No. 24 Marigold Lane, Stamford Bridge in the 1970s where a Time Lord agent was keeping the Heavenly Paradigm under lock and key. When the Master and Cole found the address, they discovered a seemingly normal suburban house, but found that the neighbourhood around it were guarded by the local residents, who were all unknowingly being mind-controlled by the Time Lords into attacking any one who got too close to the house. The two of them managed to seek refuge inside the house, but they were soon confronted by a Time Lord agent, who was guarding the facility. Said agent, posing as a harmless old lady who lived in the house, offered Cole and the Master tea laced with a sedative; Cole fell for the bait and fell asleep after drinking the drugged tea, but the Master, having figured what the agent was trying to do, didn't. Before he knocked the Time Lord agent out, the Master revealed that he needed Cole unconscious for the final part of his plan, so the tea had proved rather convenient. As Cole was unconscious, the Master strapped him to the Paradigm.
When Cole came to and asked why he was chained up, the Master revealed that the Paradigm was powered by temporal energy. By plucking Cole out from time and taking him with him in his TARDIS, the Master had averted Cole's original fate which was to die in the Dalek attack on Arcking, which had created a paradox. Cole's subsequent actions on the farming planet had saved the local population, at the cost of turning them into a new, aggressive race, when they were all originally meant to die from the fallout; which created not only a paradox, but a paradox powered by another paradox. The Master had in fact groomed Cole into becoming a nexus for a potent amount of temporal energy, which would be enough to power the Paradigm. The Master then prepared to sacrifice Cole to the Paradigm as his usefulness had come to an end, offering him some cold comfort by telling him that - with his erasure from existence - the race he had inadvertently created would also cease to exist and so would their campaign of genocidal warfare. When Cole, as his last act, protested against this callous treatment, the Master coldly reminded him that he previously had agreed to make "any" sacrifice in exchange for his help. (AUDIO: The Heavenly Paradigm)