Mark was behind the building sneaking a cigarette when the Third Doctor slipped sideways in time from his Earth to Mark's world. Mark experienced a dimming of vision and nausea, and discovered that the Inferno Project drilling had changed pitch. He then discovered that he was stuck experiencing the same day over and over, with nothing moving except time and himself.
After five years of living like this, Mark saw Helen Martin moving nearby. They approached each other, and she informed him that they had only five more years of living like this. She then faded away. (PROSE: Still Lives)