Having lived a life in the military, specifically UNIT, Sir Alistair Gordon Lethbridge Stewart had died peacefully and in bed. However, an old enemy of his and the Doctor’s, now in female form and calling herself Missy, had taken his body and converted him into a Cyberman. However, he kept his emotions and helped the Doctor thwart her plans yet again.
After leaving for parts unknown, he fell through a time-hole and discovered Rusty the Dalek alongside humans. He did something contrary to his usual man of action attitude and talked with them. After successful negotiations, he and Rusty formed a new UNIT, the Unified Nebular Intelligence Task force. Now, he fights as Tech support and Military advisor for the Vortex Riders.