A is for... Arnold, Staff Sgt.

A member of the army unit assigned to the Goodge Street fortress when London was hit by a strange dense fog. Anyone venturing into this was later found dead, smothered in a choking web substance. The city centre was evacuated. The fortress was constructed during World War II, and was built into the side of the London Underground network. The web began to take over the Underground. Arnold discovered Jamie and Victoria wandering in the tunnels and took them to the fortress, where they were reunited with Professor Travers, whom they had met in Tibet some decades before. He told them that the Great Intelligence had once more decided to attack the Earth. A new generation of Yeti were roaming the tunnels and the streets above. At one point Arnold went missing with his troops. He reappeared later, the sole survivor.
It soon became clear that someone amongst the fortress personnel was under the influence of the Intelligence - acting against the Doctor's efforts to defeat the alien entity. They planted Yeti control models in people's pockets - targeting them for attack - and they opened the fortress doors to let the web enter.
The Intelligence wanted to drain the Doctor's knowledge. He reprogrammed a Yeti sphere and placed it in a captured Yeti, which he planned to use to help him to reverse the draining machine - so that it would be the Intelligence that was drained.
At Piccadilly Underground station, the Doctor and his friends finally met the person who had become host to the Intelligence - and this proved to be Arnold. Failing to tell his companions what his plan was, they attacked the Intelligence's machine. It was ejected into space, and Arnold was killed - his body burned up.

Played by: Jack Woolgar. Appearances: The Web of Fear (1968).