The last story in Doctor Who's tenth season, this story of environmental issues was one that I never saw on TV. It was rated above "General Exhibition", so Australia's ABC didn't show it. So my first experience was with this, the novelisation by the script's author, Malcolm Hulke.
UNIT is called in to investigate the death of a Welsh miner, who turned green before he died. Jo Grant is, instead, attracted to the cause of the Wholeweal Community, who are seeking environmentally sustainable methods of living under the guidance of Professor Cliff Jones, the youngest winner of the Nobel Prize. The Doctor, however, takes a trip to holiday on Metebelis Three.
Some holiday! After being attacked by all kind of creatures, he returns to Earth to receive a message from the Brigadier, and heads off Llanfairfach to join UNIT. While Professor Jones is right top criticise Global Chemicals, even he has no idea of exactly what the company is up to.
While this story is neither particularly good or bad, and rather predictable, I think you are better off watching it on video as the final scenes, the farewell to Jo Grant as the Doctor's assistant, is very moving because of the great acting.