When the base was taken over by Ice Warriors, they ordered him to repair the damage Osgood had done to T-Mat. Fewsham sided against fellow technicians Phipps and Locke, suggesting that they should cooperate through fear of death.
Locke attempted to warn Earth of the invasion and was killed. Phipps escaped from the Ice Warriors leaving Fewsham alone under the orders of Slaar, the Ice Warrior leader.
Fewsham managed to partially repair the T-Mat system so that it allowed the transportation of several technicians to the Moon. The group was led by Gia Kelly, and she was unaware that the Ice Warriors were in control. Fewsham told them that the faults in T-Mat were a result of Osgood going mental. Under this pretence, they repaired the T-Mat. The Ice Warriors used the T-Mat to send their Martian Seed Pods to Earth.
After the Second Doctor was rendered unconscious, Slaar ordered Fewsham to T-Mat the Doctor into deep space. Fewsham was reluctant to kill a man but Slaar told him he had doomed his race by helping send the pods. He then obliged. However, the Doctor ended up in a different area of the base, likely as a result of treachery by Fewsham.
Fewsham distracted an Ice Warrior while Zoe Heriot increased the heat. However, the Ice Warrior became aware of her presence. Zoe asked Fewsham to help her and he reluctantly obliged, attacking the Ice Warrior. He was quickly knocked aside but the heat incapacitated it anyway.
Fewsham then fled with Zoe to regroup with Kelly, the Doctor and Jamie McCrimmon. Fewsham tried to explain his motives in siding with the Ice Warrior and Kelly told him that he would have to appear before an inquiry. Fewsham prepared a T-mat to return them to London, saying he would use the time delay switch to send himself afterwards. Back in London, Kelly found that no such function was in working order and so Fewsham never intended to return.
Instead he returned to Slaar, telling him that he had refused to return to Earth as he would have to face a trial. Slaar asked him if he hoped to live longer by siding with him.
Slaar ordered him to activate a transceiver to contact the Grand Marshall. While Slaar talked with his leader, Fewsham activated a video feed to Earth allowing them to hear the plans of the Ice Warriors, and crucially, a weakness of their fleet. Slaar noticed his betrayal and Fewsham told him that everyone on Earth now knew his plans. Slaar then killed him. (TV: The Seeds of Death)