The titular box is either the cabinet used by a stage magician for the vanishing lady trick, or a casket supposedly stuffed with healing crystals by a quack Doctor (each, of course, being deployed by an illusionist of one sort or another).

There’s a good solid central plot here around eavesdropping and smuggling military secrets (albeit just slightly overcomplicated by the ruse used to transmit them). It’s interesting to see pseudo-science being a concern in the early 1960s (and if it seem implausibly that a senior military man might be fooled by such nonsense, let’s not forget the dowsing bomb detectors, still in use in Iraq today almost a year after James McCormick was convicted of fraud). And, of course, it’s not the general who believes the Doctor, it’s his daughter, who shows exactly how open minded she is when Steed prepares to confront her, declaring “I am determined that he [her father] shall be cured – and by the methods I believe in.”

Steed asks if she would be willing to consider the evidence:

“Look, if I was able to prove to you that Dr Gallam was a fraud, would you believe it?”

“I could never believe such a thing.”

So alternative medicine was a 60s concern? People wouldn’t be so blinkered now, though, of course. Just don’t Google healing mineral radiation…

Meanwhile, this is the first Venus Smith since The Removal Men eleven episodes previously, and she is absolutely delightful in this. Once again, she’s a complete innocent in Steed’s game – she’s got a gig at the club where the magician’s assistant was murdered and she doesn’t know it was Steed that got her it (even though the manager does!) But she’s happy to have work, and rather than using her as bait or something else equally ruthless, Steed seems to have planted her simply as an extra pair of eyes and ears. She grins and bubbles her way through the episode, eagerly investigating between numbers, and for once she doesn’t get put through the wringer. Although she’s not an Avengers girl in the Gale/Peel/Purdey mould, I have become quite taken with her. More Venus, please!