The Diabolical Masterminds of Series 6 (1968-9)
Monte Bristow, Britain's "games king" has built an empire on board games and puzzles, but his past was much murkier - he had been arrested and tried for blackmarketing, as Private Daniel Edmund of the BAOR years before - and sought to avenge himself by killing those who had had him imprisoned.
The villainous Charles Lather of Classy Glass Cleaning won the concession to clean the windows of Britain's Ministry of Top-secret Codes (MOTC) and quickly discovered he could gain access to the latest cyphers and key phrases by knocking out and brainwashing the staff. The company subsequently lost the concession - presumably because they stopped cleaning the windows altogether while stealing secrets - but kept turning up every week and purloining more information.
Dr Glover conspired to take over the world by disseminating a highly contagious and fatal strain of the common cold - so that no-one could stop him, he resolved to kill of the world's ear, nose and throat specialists, but this proved his undoing as he and the matron, whose staff of nannies sent out the poison pen missives, didn't cover their tracks at all.
Hinnell and Constantine enabled the immobilised Boris Kartovski to infiltrate British Security by kidnapping security staff and conditioning their brains to be receptive to takeover by Kartovski - his thoughts in place, he killed security personnel and instilled fear and loathing in the department.
Computer genius Sir Wilfred Pelley is imprisoned in his own house by enemy agents who seek to destroy British Security's main computer and earmark Pelley as a traitor. Drugged and confused, Pelley still manages to send a mathematically encrypted SOS and his captors - Loris and the butler are captured, and the saboteur Tobin is king-hit by Steed.
Society blackmailer Lord Edgefield, facing prison upon the completion of a dossier into his nefarious activities, hires the services of Sykes and his Dreemykreem Dairies to thwart the judiciary's case. Sykes has developed a serum that compells the consumer to lie, and not realise they've done it, and he and his minios dispatch the drug in milk bottles to the witnesses involved in the case, make them all retract their testimony.
Barlow and Withers have been kidnapped and replaced by impostors who use the Colonel's lighthouse to spy on the nearby Carmadoc scientific research facility. The real Colonel had developed a method for transmitting and receiving audio waves through light beams and being able to use any reflective surface to spy on the surrounding area. The arrival of journalist Pandora Marshall and Tara at the lighthouse exposed the plot and they were apprehended after a very physical tussle.
Henley Farrer owned the world most expensive pearl, and knew there was a group of Dashiell Hammett enthusiasts who all wanted it for themselves. In order to rid himself of these pestilent dogs, he faked his own death and had his butler, Zoltan, entrust Steed with the 'key' to its location, an ornate dagger that was apparently cursed. Sure enough, the hounds cam hunting, and many were killed in the process but Farrer was undone as Steed deduced the location of the jewel and when he revealed himself to be still alive, the pearl was destroyed and so was he.