Steed impersonates the forger Riordan, which allows him to join a crime syndicate that prints forged banknotes. Having uncovered their plans, he finds his every move being watched by The Cardinal - the second-in-command. Only the pretence of an injury eventually allows Steed to visit his Doctor's surgery and Keel ensures that The Cardinal and his superiors, Bloom and Hooper are arrested.

Cast & crew * mug shots in sepia denote images sourced from other productions.

crew
John Steed	Patrick Macnee {New Avengers}{007}
Five	Heron Carvic {Police Surgeon}{Dr Who}
Secretary	Cynthia Bizeray {regular}
Bloom	Michael Robbins {regular}{Dr Who}{The Saint}{Adam Adamant}{Callan}{Department S}{The Sweeney}{Pink Panther}
Hooper	George Murcell {regular}{Dangerman}{007}{The Professionals}{The Saint}{Jason King}{Randall and Hopkirk}{The Champions}
Warren	Vic Wise {regular}
The Cardinal	Alex Scott {regular}{Dangerman}{The Saint}{Space 1999}{Randall and Hopkirk}{Department S}{Jason King}{Adam Adamant}
Lisa	Delphi Lawrence {Dangerman}{The Saint}
Carol Wilson	Ingrid Hafner {Police Surgeon}{regular}
Dr David Keel	Ian Hendry {Dangerman}{New Avengers}{Police Surgeon}{The Saint}{The Sweeney}{regular}
Teleplay by	Richard Harris {The Sweeney}{The Saint}{Adam Adamant}{Police Surgeon}
"The Avengers" Theme composed and played by	Johnny Dankworth
Designed by	Patrick Downing
Producer	Leonard White {Callan}{Police Surgeon}
Directed by	Don Leaver {Police Surgeon}

The Avengers

Season 1, 1961 - Episode 3

Square Root Of Evil

by Richard Harris

Cast & crew * mug shots in sepia denote images sourced from other productions.