A top-secret chemical formula has been stolen by STENCH (the Society for the Total Extinction of Non-Conforming Humans). Fearful of what would happen if that formula fell into the wrong hands, the Chief of the Secret Service reluctantly sends the only agent he has left, the bumbling and snide Agent Desmond Simpkins (Kenneth Williams), and his three trainees, Agent Harold Crump (Bernard Cribbins), Agent Daphne Honeybutt (Barbara Windsor), and Agent Charlie Bind (Charles Hawtrey), to find the formula. Daphne has a photographic memory.The Agents travel separately and in different disguises, to Vienna, where they contact Carstairs (their local contact following STENCH) at the Cafe Mozart. They are watched by the STENCH agents, while Simpkins manages to knock out Carstairs (Jim Dale) while Carstairs was following Milchmann (Victor Maddern ) who had the formula on him.Carstairs found that Milchmann made a deal with Dr Crow (Judith Furse) (who heads STENCH) to hand over the formula for $50,000. Meanwhile Dr Crow instructs his henchman, The Fat Man (Eric Pohlmann) to kill Milchmann and to retrieve the formula from him. From the clues in the cafe, the 4 British agents travel to a warehouse in Vienna, where the Milchmann is supposed to meet The Fat Man for the exchange. They find Milchmann fatally wounded, who tells them to head to Algiers to apprehend Fat Man.The British agents contact their HQ and Simpkins tells them that Carstairs never met them in Vienna. The secret service directs Carstairs to meet the team in Algiers before they go to apprehend Fat Man. The British team tracks down the Fat Man at a local cafe in the Street of a Thousand Artisans. Daphne and Harold attempt to steal the formula back whilst disguised as dancing girls in Hakim's Fun House, where the Fat Man is relaxing. On the orders from HQ, Carstairs also reaches the same place & takes the formula from the Fat Man at gun point, when Daphne and Harold were in the room with him. Before Carstairs can explain who he is, Simpkins knocks him over & they seize the formula. The agents also encounter the mysterious Lila (Dilys Laye), whom they are uncertain if they can trust.Dr Crow instructs Fat Man to join the British agents on the Orient express to steal the formula back. He reveals that Lila works for him & will assist the Fat Man in outwitting the British agents. Lila & Fat Man manage to capture the British agents, but not before they destroy the formula & only Daphne has taken a photographic memory of it.The three agents end up captives of STENCH; Daphne is interrogated by the evil Dr Crow, head of STENCH, but she fails to subdue any of the agents. The British agents escape but are caught up in an underground automated factory process, from which they only escape when Lila pulls a gun on Dr Crow. The group are transported by lift to the surface, which is revealed to be the office of the Chief of the Secret Service; the headquarters of STENCH are below the streets of London. Lila reveals to Simpkins that she is a double agent, working for SNOG - the Society for Neutralising of Germs. The STENCH headquarters self-destructs, presumably taking the Secret Service with it.