The 10th season comes to a close with this story written by Robert Sloman, this six part story ran from 19 May to 23 June, 1973.
UNIT is eager to assign The Doctor (Pertwee) and Jo (Katy Manning) to investigate a strange death at a mine in South Wales, the body is apparently glowing green. Now that the TARDIS is repaired however, The Doctor is less inclined to stay on earth for extended periods of time, so Jo sets off to investigate, while The Doctor takes a trip on his own, and ends up in a bit of trouble before finding his way back.
Jo and a miner end up getting trapped in the complex of tunnels, as The Doctor returns from his trip to late to catch up to her. Following her down, they discover not only a weird green pollutant, but that the effect it is having on some of the insect life is frightening, as the come across a colony of massively oversized maggots. The folks in charge at Global Chemicals, as well as their big Boss, are trying to keep it all quiet, going so far as to brainwash and control their own employees and any interlopers they feel may cause them problems, even UNIT’s own Captain Yates (Richard Franklin).
Once free of the tunnels, Jo pairs up with a young scientist, Cliff Jones (Stewart Bevan) and it’s rather obvious that there is an immediate attraction between the two. The Doctor doesn’t have time to notice as he’s slipping into Global Chemicals to touch base with Yates, and find out what he’s learned and who in the company is actually responsible for letting the pollution get to the point that it has.
This involves a rather charming sequence, that provides some nice comedic moments, as The Doctor disguises himself first as a milkman, and then a cleaning lady. Yates seems to have trouble interacting with The Doctor, though the Time Lord rouses him eventually, but also learns that everything is being run by a conniving computer, the Boss. The Doctor out thinks and out smarts the computer, however and escapes…
Just in time to save Jones and Jo from an attack by the giant-sized maggots!
Through the course of the story, he saves Jo, Jones and Yates more than once, though it is thanks to a mistake that Jo made during one of Jones’ studies that leads to a way to stop the maggots before most of them evolve into their next state, though at least one became a giant fly.
Still, by story’s end, everything is wrapped up nicely, and balance is restored, happily. And speaking of happy, it is time for Jo to take her leave from the TARDIS and travelling with The Doctor, as she and Jones plan to go off together, with plans to get married.
As the season ends, we get a really good glimpse of the Lonely Doctor motif, as he drives off, alone in Bessie.