For the Doctors next trip, he plans to take Jo Grant to Metebelis Three, a beautiful 'blue planet'. But when the Brigadier requests the Doctor and Jo for assistance on a new project, Jo opts to follow the Brigadier despite the Doctors reminder he has a time machine and can be back before they even left. When Jo is insistent, the Doctor rushes off in a huff by himself to search for a famed blue sapphire, but discovers a less than hospitable planet.
Meanwhile, the Brigadier and Jo are off to visit Global Chemicals. The Brigadier has been charged with keeping peace and security, but Jo would rather visit Professor Jones who is speaking out against Global Chemicals and all the pollutants it produces for the Earth. Jo Grant falls immediately in love with him and his principals. In turn, Jones is smitten with Jo, despite her clumsy tendencies.
Things turn from bad to worse when miners start turning green and dying. Professor Jones believes it's the waste effects from Global Chemicals. For safety reasons, the mine is closed and an attempt is made to halt production at the factory but Stevens, the director, refuses to do so. Employees who side against him mysteriously become zombie-like loyalists or kill themselves.
After some investigative work on UNIT's behalf and the Doctors own infiltrations, its soon discovered Stevens is not the true director. He is actually a pawn for a sentient computer known as B.O.S.S. (Biomorphic Organisational Systems Supervisor). Boss's initial primary function was to maintain profits for Global Chemicals, but now seeks to enslave 'inefficient' mankind.
BOSS isn't the only problem. Another result of the Global Chemicals toxic waste are large maggots capable of infecting humans with the 'Green Death'. Problems intensify when it's discovered they can cocoon themselves and turn into flying dragonfly-like creatures, capable of spitting the venom. The day is solved when it turns out a special fungus grown by Professor Jones is deadly to the maggots.
BOSS is also destroyed when the Doctor is able to free the mind of Stevens from the computer, who sacrifices himself in order to destroy BOSS. Crises averted, the Doctor wishes to wisk Jo off on another adventure but is shocked to learn she has decided to leave with Professor Jones and get married. Somewhat heart-broken, the Doctor gives her the blue crystal he acquired on Metebelis 3 as an early wedding present. As Professor Jones' friends and the UNIT officers celebrate Jo's engagement, the Doctor slips off quietly.
WHO: Jo Grant
DEPARTURE LOCATION: Earth, circa 1973
REASON FOR DEPARTURE: Falls in love with Professor Jones, and decides to travel the Amazon with him.
NEXT: Time for Sara Jane Smith