Johnstone was a life sign specialist who helped Mirana after the link to the Dalek Emperor was detected. He managed to stop the link, but he failed, however this proved useful as it told the Earth Alliance where all the Dalek positions were on the planet. (AUDIO: Project Infinity)
During the Alliance-Dalek war, he helped Mirana to unfreeze a cyro-pod. It contained Susan Mendes. (AUDIO: Dalek War: Chapter One)