Third Doctor, Jo, and Brig "UNIT is called to a sleepy mining town in South Wales to investigate the mysterious death of a miner in an inactive pit. The Wholeweal environmental community believe that a local chemical company has something to do with the death, and when the Doctor heads a mission down the mines he discovers thousands of maggots surrounded by poisonous slime. Further investigation at the chemical works reveals the owner to be under the malign influence of a sentient computer, BOSS." Written By: Robert Sloman and Barry LettsDirected By: Michael Briant