John Steed's work as an agent
  Assignments and (false) identities
Episode
Assignment
Identity or name
Hot Snow
Steed infiltrates a gang of drug dealers
Steed
Brought to Book
Steed pretends to be working for Nick
Mason, head of a protection racket
Steed
Square Root of Evil
Steed infiltrates an organisation that
intends to flood Britain and Europe with
forged bank notes.
Timothy James Riordan, an Irish, born in  
Dublin, Protestant                                                                           
Steed has a problem as Riordan is only  
5,6" high while Steed is 6,2"
Nightmare
Dr Keel is mistaken for Professor
Braintree and Steed tries to find out who
is behind the attemped kidnapping of
Faith Braintree, a scientist who has
worked on a top-secret medical research
project
Steed
Crescent Moon
Steed has been sent by the department
to the Caribbean island of Pascala to
investigate the kidnapping of  Carmelita
Mendoza, daughter of General Mendoza,
a  pro-Western politician.
Steed
Girl on the Trapeze
Steed does not appear in this episode.
Steed does not appear in this episode
Diamond cut Diamond
Steed is ordered by his superior One-Ten
to infiltrate a gang of international
diamond smugglers.
John Ryan, an airline steward
The Radioactive Man
Steed and Keel are trying to find a man
who has accidently taken a radioative
pellet from a research laboratory.
Steed
Ashes of Roses
Steed investigates a case of arson and
insurance fraud.
Steed
Hunt the Man Down
Steed, who is under orders to pursue the
former prisoner Preston and to recover £
100000 from a former robbery.
Steed
Please Don't Feed the Animals
Steed has to find out if the civil servant
Felgate, who works with with codes and
ciphers, has become a security risk.
Steed is posing as recruit at Felgate's
office
Dance With Death
Steed has to proof Dr Keel's innocense
after a woman got strangled with the
doctor's scarf.
Steed is posing as "Mr Rogers" at a
dance academy
One for the Mortuary
Steed is responsible that a top secret
chemical formula gets to Geneva.
Steed
The Springers
Steed and Keel have to trace an escape
route for inmates at a prison in the South
of England.
Steed poses as a warder at a prison.
Steed poses as the widowed Navy
Commander Kenilworthy
The Frighteners
Steed has to deal with the criminal
underworld with a group of so called
"frigtheners".
Steed
The Yellow Needle
Steed is send to Tenebra, a state in
Africa to prevent another attack on the life
of Wilberforce Lungi, Prime Minister of
Tenebra.
James Sanderson, a journalist of the
Daily Globe
Death on the Slipway
John Steed investigates the death of a
fellow agent who was working undercover
at a government dockyard where a
nuclear submarine is under construction.
Steed
Double Danger
John Steed works on behalf of an
insurance company, trying to find uncut
diamonds worth £200000 that were
stolen a jewellers.
Steed
Toy Trap
Steed brings down a vice ring of
enforced prostitution..
Steed
Tunnel of Fear
Steed investigates at a funfair how top-
secret information have been leaked to
foreign countries.
Steed works undercover as a funfair
barker
The Far Distant Dead
no Steed in the episode
no Steed in the episode
Kill the King
Steed had been assigned to guard King
Tenuphon, who is threated with
assassination by dissidents.
Steed
Death of Winter
Steed fights a neo-fascist party called
Phoenix.
Steed
The Deadly Air
Steed investigates a sabotage in a to
secret lab that develops an important
vaccine.
Steed
A Change of Bait


Steed and Keel are investigating against
a group of fraudsters,arsonist and
insurance fraud.
Steed
Dragonsfield
Steed investigates murder and sabotage
at a government facility where scientist
are experimenting with materials
intended to protect astronauts from space
radiation.
Steed
[Series 2][Series 3][Series 4][Series 5][Series 6]
Series 1
[Dreams, Nightmares and Memories][Home][Steed's Superiors]
Most of the crimes which Steed is investigating in series one are down-to-earth: smuggling, blackmailing, he has to deal with forgers, drug dealers, insurance fraud, robbery and so forth - after all more police work than tasks for a secret agent. Only a few episodes such as Tunnel of Fear, Please Don't Feed the Animals, One for the Mortuary, Dragonsfield, Death on the Slipway or The Deadly Air call for an agent of "The Department" or "The Ministry", the mysterious organisation Steed is working for.
Best information about John Steed's assigments in series 1 can be found in this book:

Richard McGinlay & Alan Hayes: With Umbrella, Scotch and Cigarettes. An Unauthorised Guide to The Avengers Series 1, Hiddden Tiger 2014