This was the other episode of The Avengers that I'd heard of before I started this 'thon. And it's even more similar to its Doctor Who counterpart than The Cybernauts.
A plant that came from space (ahem) has mind-control powers (cough) and is influencing Sir Lyle Petersen, a plant fanatic (yep) to bring it to germination (aha). Our heroes are besieged by the plant in a country house (right), and deal with it using pesticide. The RAF don't have to bomb the place although they are involved in the discovery of the seed. (They don't function in the story like UNIT do, but they certainly reminded me of them).
Further similarities to The Seeds of Doom are seen in eccentric old lady Dr Sheldon (though she's a botanist here) and the mistaken initial assumption that there was only one extraterrestrial seed.
I enjoyed this one though I thought that the scenes inside the house, with Steed battling a mind-controlled Mrs Peel to deploy the pesticide, went on too long. Though perhaps that was just tension induced by the number of times he almost manages to use the chemical, only to be stopped at the last second. It was interesting that he only succeeds in putting her out of action when they accidentally bang their heads together - presumably it wouldn't have looked good to have him purposely knocking her out.
Athene Seyler (Dr Sheldon) returns from Build A Better Mousetrap. Derek Farr (Sir Lyle) was the original voice of Orac. Also seen: Edwin 'the landlord in Reign of Terror' Finn, Harry Shacklock returning from The Mauritius Penny.
