In fog-wreathed London, Long Harry (Philip Latham), a denizen of the city's criminal underworld, finds himself pursued by a young man on a motorbike (Dudley Sutton, who'd recently played a much friendlier biker in kitchen sink drama The Leather Boys), eventually making his way to a phone box and interrupting Simon Templar in the midst of a monologue about the uselessness of weather forecasters.
Long Harry makes it to Simon's pad and pours out his troubles. The biker is employed by a mysterious villain known as the Scorpion, for whom Harry recently broke into the house of Mark Deverest, an official at the Ministry of Aeronautical Affairs (as we don't get quite enough about aeronautical affairs around here), and stole a letter. He was instructed to leave the letter in a locker at Victoria station and give the key to Patsy Butler, a hostess at a nightclub called the Birds Nest. Suspecting the letter was going to be used for blackmail, which he hates (so why did he steal it?), Harry hid at Victoria and saw the Scorpion retrieve it. But he was spotted and, as he can identify the blackmailer, now has his henchman out to kill him.
Simon dashes out in Harry's hat and coat to distract the biker. There are many guffaws to be had from the comparison between the biker in close up (Dudley Sutton) and long shot (not Dudley Sutton). Their hairlines are very different.
The scene shifts to the home of Mark Deverest (Ronald Leigh-Hunt), who's in a confab with his lawyer Garniman (Geoffrey Bayldon) and his secretary Karen Bates (Catherine Woodville, last seen on Tuesday in Danger Man). Simon drops in to talk to Deverest about the Scorpion situation, but Deverest is insistent he isn't being blackmailed and isn't missing an important letter.
But just then he receives a call from Patsy Butler (Nyree Dawn Porter), instructing him to come to the Birds Nest and pay up.
Karen listens in on another line, and reveals all to Simon. They head to the Birds Nest together, and are greeted by cheerful owner Montgomery "Birdie" Bird (former music hall star Leon Cortez). Simon and Karen watch as Deverest converses with Patsy. She makes it clear this is just the first instalment.
Simon follows Patsy out the back, interrupting her in the midst of exchanging sweet nothings with Eddie, the biker (Patsy and Eddie - how wonderful!). He escapes, she gets a smacked bottom from the Saint before doing likewise.
Eddie heads to the Scorpion's lair to see his boss. It's filled with exotic animals, including several examples of the baddie's namesake.
Long Harry's been at Simon's place the whole time, but isn't likely to mind the wait because, as Simon discovers when he returns, he's been stabbed to death. Deverest, meanwhile, is quizzed by his wife (Eve Lister) about what's going on, and he confesses all. Disappointingly, he's being blackmailed over a business matter: before taking up his current job he was meant to divest himself of all shares in aerospace companies. The letter that's been stolen reveals that he hasn't.
For once it's Simon who calls in Inspector Teale to help. The lugubrious policeman has little sympathy with victims of blackmail: "If the public had a little more guts it would help our job a great deal." He initially wonders if Simon's fallen victim to the Scorpion, getting very excited at the prospect.
Simon pays a visit to Bird, who denies all knowledge of the Scorpion's activities, and claims not to know much about Patsy either. Simon sets to bribing the Birds Nest staff for information.
Securing Patsy's address, Simon heads over to see her. It's been a long time since a Tretchikoff painting's turned up round these parts, so I was very happy to see one adorning the grimy walls of Patsy's flat - not the especially ubiquitous Chinese Girl this time but the very similar Lady of the Orient. Simon's told very plainly to go away, and realises that Patsy's involvement in the gang is purely down to her love for Eddie. He warns her the biker's a dangerous character, but it falls on deaf ears.
Deverest admits to Garniman that he's being blackmailed, and the lawyer appears to be very disappointed in him. Eddie follows after Garniman when he leaves the Deverest residence, and it emerges that Garniman is in fact the evil Scorpion. He talks Eddie into disposing of his girlfriend.
Eddie heads to Patsy's flat, and attempts to strangle her with a stocking. She manages to break free, but falls headlong down the stairs, where she's discovered by her landlady (Hilda Barry) as Eddie makes a sharp exit.
Simon finds a mysterious package when he arrives home. It's a cigarette box containing a deadly scorpion, which he only just manages to kill before it can sting Karen.
Patsy survived the fall, but as she tells Karen when she pays a visit, "My dancing days are over." Patsy gives Karen the Scorpion's contact details, but when she leaves the hospital Karen finds Garniman waiting for her.
Simon and Eddie have a ding-dong in Bird's office, with Simon getting the better of his foe and setting off on his bike to the Scorpion's residence.
Realising he's been defeated, Garniman uses one of his own scorpions to poison himself.
The Scorpion continues The Saint's upward trend: it's gritty, tremendously exciting and unexpectedly violent, and Porter, Sutton and Bayldon all give brilliant performances. Quiz time again: does anybody recognise this uncredited actress playing Patsy's nurse? I'm sure I should, but I can't quite place her. Your help, as ever, would be greatly appreciated.