Production Code: BFPAVENGE001


Steed is assigned by one of his superiors, '5', to infiltrate an organisation that intends to flood Britain and Europe with forged bank notes. What is not known is when or how the notes are to be distributed. Another agent, Tobert, had been working under cover among the gang, but he was discovered and brutally murdered. Fortunately, a forger called Riordan is due to be released from prison. '5' knows that the counterfeiters will require Riordan's services and have probably never met the man before, which means that Steed can take his place.

Following three weeks of training in the finer points of forgery, Steed assumes the role of Riordan. He is welcomed by the gang's boss, Hooper, but the second-in-command, a tough customer known as the Cardinal, is far less friendly. The Cardinal is deeply suspicious of the newcomer, and tries various tricks to test his loyalty. Breaking into Hooper's safe late at night, Steed injures his hand and is almost discovered by Hooper and the Cardinal. The injury affords Steed the opportunity to visit Dr Keel, whose assistance may be required later. However, when the order is finally given for the money to be distributed, the gang might not allow Steed to see the doctor a second time...

Click here to read about the original television episode

The Avengers - The Lost Episodes:
Volume 1, Episode 3
Recording Dates:
23-25 July 2013
Recorded at: Moat Studios
Duration: 42 minutes 47 seconds


Released as a part of
The Avengers - The Lost Episodes,Volume 1
Release Date:
Wed 8 Jan 2014
Physical Release: Audio CD
Download Release:
MP3 / M4B Formats
Dr David Keel
John Steed
Carol Wilson
The Cardinal
Prison Guard
Anthony Howell
Julian Wadham
Lucy Briggs-Owen
Phil Mulryne
Beth Chalmers
Tim Bentinck
Alan Cox
George Rainsford
Kieran Bew
Blake Ritson
Sophie Aldred
Colin Baker

At the present time, no original soundtrack has been released by Big Finish.


Interviews with actors Anthony Howell, Julian Wadham and Lucy Briggs-Owen, and adaptor John Dorney;
Production Notes Booklet (with CD only)


Purchase from Big Finish


Writer Richard Harris, from a story by John Bryce
Adapted for audio by -
John Dorney
Recording, Sound Design, Music and CD Mastering
Toby Hrycek-Robinson
Series Theme -
Johnny Dankworth, rearranged by Toby Hrycek-Robinson
Interviews edited by -
Jamie Griffiths
BFP Administration -
Miles Haigh-Ellery, Charlotte Baker and Alison Taylor
Producers' Assistants - Hannah Peel, Paul Spragg and Frances Welsh
Cover Illustration -
Anthony Lamb
Booklet Design -
Mark Plastow
Booklet Notes -
Richard McGinlay
Web Services -
Hughes Media
Marketing Consultant -
Kris Griffin
David Richardson
Executive Producers -
Nicholas Briggs and Jason Haigh-Ellery
Ken Bentley

Thanks to Massimo Moretti, Brian Clemens, Sam Clemens, Marcus Hearn, Richard McGinlay and Toby Hrycek-Robinson

A Big Finish Production


  • Production Brief... This audio play is based on the television episode Square Root of Evil, originally broadcast third in the series' run on Saturday 21st January 1961 at 10.00pm in the ABC Midlands and North ITV regions. Due to the slow take up of the series in other ITV regions, this episode was never transmitted in any other part of the United Kingdom.

  • The television version of this episode does not survive today. However, one camera script has been located, along with a modest number of production photographs, comprising shots of empty sets and publicity stills of Ingrid Hafner as Carol Wilson.

  • The audio adaptation of this episode was recorded over a three day period between Tuesday 23rd and Thursday 25th July 2013 at Moat Studios. Four episodes were recorded during these sessions: Hot Snow, Brought to Book, Square Root of Evil and The Radioactive Man. The latter would be held back for the second volume.

  • Brought to Audiobook... As John Dorney comments during the bonus interviews, the camera script for this episode is rather lacking in detail when it comes to the visual aspects of the story. For example, a film sequence at the top of the episode is described simply as "OPENING TEASER (Car driving along country lane, etc.)", while the fight scene at the end is covered by just two unhelpful words, "As directed." Therefore, some educated guesswork was required. The audio production begins with the killing of Tobert (Blake Ritson) in said country lane and the dumping of his body. It is unlikely that there would have been any dialogue during this scene in the television version (the camera script indicates that the film insert was silent), but this is audio, so some spoken lines are necessary. Dorney deduces that the closing fight would have ended with Jackie Warren (here played by George Rainsford) using Lisa (Sophie Aldred) as a human shield, which is why he seems to be the last man to be subdued by Steed and Keel: "All right, Jackie," says Steed in the script, "the party's over."

  • Trivia... This episode gives us our first view in the series of one of Steed's overlords. '5', played by Heron Carvic, was the first of Steed's superiors to be depicted on screen, though no visual record of this character is known to survive today. Douglas Muir would make his debut as One-Ten later in Series 1, his first confirmed appearance occurring in Diamond Cut Diamond, possibly stepping in because Carvic was unavailable to reprise his role. The plot details of the intervening episodes Nightmare and Crescent Moon are sketchy, and their cast information is quite possibly incomplete, so one cannot rule out the possibility of a reappearance by '5' during these episodes, or an appearance by One-Ten prior to his commonly accepted debut in Diamond Cut Diamond. It might have been tempting for the audio producers to carry out a bit of retroactive continuity and replace '5' with the more familiar One-Ten, but commendably Dorney has stuck to the original script. Phil Mulryne plays the part of '5' in the Big Finish adaptation of Square Root of Evil.

  • Stop Press... An interview feature written by Paul Spragg entitled Trusty Steed appeared in Vortex Issue 59 in January 2014. Actor Julian Wadham was the interview subject, and the feature concentrated on his first impressions of playing John Steed. Asked about the relationship between Steed and Keel, Julian replied, "Well, it's early days. It's a bit like Holmes and Watson. It's rather like going mining or something; you're going right back to the very lowest level where the role was being created for the first time and you have to sort of feel your way into it. It's very interesting watching its evolution. I don't know where it'll go from here; this is the early stages."

    Read the rest of the interview by downloading Vortex Issue 59 from Big Finish


  • And Finally... The events of this episode have an unusually long duration. Approximately three weeks pass between Steed's initial briefing and the resolution of the case. Most of this time elapses between the records room scene and the sequence outside Wandsworth prison, with Steed researching in the intervening time the forgery racket and the man he is to impersonate. A similar two-week break in the narrative occurs in Dead of Winter, to allow Keel to study Dr Fischer's character and history. A modern interpretation of the time lapse in Square Root of Evil might be to place the opening briefing scenes before Hot Snow, with Hot Snow and Brought to Book taking place during the intervening three weeks. This could account for the particularly shady nature of Steed during these opening episodes. In Brought to Book, the agent states that he is "Right under cover" and that "if I once give evidence in a court of law, my usefulness is over."

Plotline by Richard McGinlay Declassified by Alan Hayes with Richard McGinlay
Big Finish Productions Reproduced with permission

With thanks to David Richardson, John Dorney, Mark Plastow
and Big Finish Productions for their kind assistance


Back to Top