In The Case of the Flying Clown, Jacob Ironmaster was a weathy American arms manufacturer with a reputation as a warmonger. For a long time, Ironmaster was a mystery man to American and British authorities and little was known about him. It was only when Biggles joined the "Air Thrills Unlimited" flying circus undercover and began his investigations that a fuller picture of Ironmaster began to emerge. In particular, Biggles discovered that Ironmaster was in fact leading a double life as "Nemo", the clown and main act of the circus.
Before Biggles' discoveries, all that was known of Ironmaster was that his parents were originally in the show (i.e. circus) business in Ireland. Later he had become an industrialist and an armaments king. He made his fortune from war and would do his best to start them. He was seldom seen in public probably because he was believed to have had an accident in his youth which deformed him for life. After the Second World War, he became the main financial backer of "Air Thrills Unlimited", an international air circus. The circus was due to open in Paris by performing for the delegates of the World Peace Conference. This excited the curiousity of the British and American security authorities because, as Biggles put it, "Ironmaster and the Peace Conference don't mix." Wing Commander Jimmy Strickland was sent in undercover as a stunt pilot but was killed in an aircrash. Biggles was sent in next, together with the rest of his crew, posing as mechanics.
Biggles was especially concerned about the activities of the clown Nemo whom Stickland's ex-mechanic Jones described as a "ropey type" whom everybody disliked and who never seemed to take his makeup off. Jones had seen Nemo behaving suspiciously near Strickland's aircraft after it crashed, and the wreckage went up in flames shortly thereafter. Strickland's last words to Jones was also to warn him that Nemo must be stopped in Paris.
After joining the circus, Biggles encountered Nemo at close quarters and realised that he was in fact Jacob Ironmaster in disguise. Paddy O'Shea, the old lioner-tamer, provided Biggles with some background about Nemo. Paddy had first met Nemo many years ago when the clown had been a "tall handsome fellar" who put on a fine high trapeze act. One day, during a show, a rope had broken and he fell, going right through the safety net. Nemo survived the fall but was permanently disfigured. Unfortunately, the audience had laughed during the fall, thinking that this was part of the act. Paddy related that he could never forget the look of hatred on Nemo's face when he heard the crowd laughing.
Prophetically, Paddy told Biggles that it would not surprise him if Nemo one day decided to get his own back on the crowd. Acting on this lead, BIggles, listened in on the radio intercom between Nemo and his pilot Nix during a rehearsal of their flying wing-walking act. Nemo was heard directing the pilot to fly over the grand stand. He was heard saying, "The scum that laughed when I went through the net won't laugh tomorrow."
With few options to intervene, Biggles decided on a desparate plan on the day of the show. Nix would be locked up under Algy's guard in his dressing room. Meanwhile Biggles, disguised as Nix, would take his place in the stunt aircraft. Once airborne, Biggles made a grab for Nemo's haversack (it turned out to contain acid which Nemo had intended to throw on the audience--the Peace Conference delegates). Nemo, realising that the game was up, whipped out a pistol and would have shot Biggles. Biggles took the only action available--he put the aircraft into a slow roll at 50 feet. Nemo lost his footing and fell off. His parachute opened but he was too low and Nemo/Ironmaster plummeted to his death.