Ian Marter

Ian Don Marter (28 October 1944 – 28 October 1986) was an English actor and writer, known for his role as Harry Sullivan in the BBC science-fiction television series Doctor Who from December 1974 to September 1975, with a non-regular, one-serial return in November and December 1975. He sometimes used the pseudonym Ian Don.

WIKIPEDIA

DOCTOR WHO · S12, E1

Robot

UNIT are called in when top secret plans and valuable equipment is stolen from high security establishments. The newly regenerated Doctor suggests that the thief may not be human.
DOCTOR WHO · S12, E10

The Sontaran Experiment: Part 2

Transported back to the Earth of the distant future from the Ark, the Doctor, Harry, and Sarah discover that they have unwelcome company. An alien warrior is conducting deadly experiments to assess the suitability of the human race as an adversary prior to a full scale invasion of Earth.
DOCTOR WHO · S13, E1

Terror of the Zygons

The Doctor, Sarah and Harry return to Earth to help the Brigadier discover what is causing the destruction of oil rigs off the coast of Scotland.
DOCTOR WHO · S13, E2

Terror of the Zygons

Sarah is attacked by a Zygon disguised as Harry who is a prisoner on the Zygon spaceship. The Doctor falls into a trap and is hunted down by the giant Skarasen.
DOCTOR WHO · S13, E3

Terror of the Zygons

The Doctor and Sarah find their way into the Zygon spaceship at the bottom of Loch Ness while the Brigadier prepares to force it to the surface with depth charges.