Dai Evans was a Llanfairfach miner.
He, Dave and Bert Pritchard went to the mine to investigate Ted Hughes's death. While down in the mine, Evans was infected by Global Chemicals pollutants and managed to call for help over a telephone to Dave and Bert.
By the time Jo Grant and Bert found him, he had almost succumbed to the toxins, but by the time Dave and the Third Doctor found him, Evans had died. (TV: The Green Death)
