UNIT is called in after a miner from the Welsh village of Llanfairfach is found dead, his skin glowing bright green. While the Doctor investigates the nearby plant of a company called Global Chemicals, his assistant Jo joins forces with a local environmental group, led by Professor Clifford Jones. They discover that the mine workings are full of giant maggots and green slime-- both lethal to touch. These have been produced by chemical waste pumped from the Global plant where the workers have been hypnotized by BOSS, a computer intelligence with a will of its own and a plan to control the world. As the maggots reach maturity, the Doctor must find a solution, using a fungal virus and a blue crystal from a planet called Metebelis Three. Ages 5 and up.