On his day of, Dangermouse decides to search for the long lost crown on Questzebottle (ancient King of the Aztecs and inventor of the practical joke) in the depths of the Amazonian Jungle. Baron Greenback decides to wait outside the temple and snatch the crown after DM has done all the difficult bits inside.
A dragon has been terrorizing the countryside of Wales. Colonel K. sends Dangermouse and Penfold to investigate. Sure enough, they run into Jones the Dragon, who's small size belies his strength. Jones, for lack of a better 'damsel', kidnaps Penfold and takes him to his castle, a gift from his 'Frogfather', Baron Greenback.
Greenback reveals his latest secret weapon in the Albert Hall: A Magnetic Molecular Molder. A machine capable of making hundreds of duplicates of anyone. The Baron plans to make a thousand copies of himself. Unfortunately, the machine gets stuck on producing Penfolds and soon the Albert Hall is filled to the brim with copies of the cowardly sidekick. Dangermouse manages to throw a wrench into the machine and solves the case in record time, mainly because the original Penfold is off to the Annual Conference of Cowards Anonymous.
Colonel K. just barely manages to inform Dangermouse and Penfold that Greenback has established himself on the moon and is trying to jam all communication. So, our heroes pack their weightless boots and jump into the Spacehopper to head for the moon.
When the United Nations inform Colonel K that aliens from another planet have announced their imminent arrival in Scotland, he sends his two top agents, Dangermouse and Penfold to meet them. Penfold describes the mechanical being they encounter as a 'clockwork mushroom'.
Baron Greenback has invented an 'Auto McCrikey' that can overtake other mechanical devices by remote control. In order to use it on Dangermouse's Mark 3 flying car, he orders his henchman, Stiletto Mafiosa, to lure their arch enemy into a trap. The Italian crow does this by threatening no 10 Downing street with an inflatable tank.
Agent 57 sends word to H.Q. that Greenback has build his latest hideaway on a pinnacle of rock in the middle of a volcanic crater in Northen Iceland. Furthermore, 57 has disguised himself as a flat battery, so the Baron can't start his Froghead flyer. When Dangermouse and Penfold arrive on the scene, they bump into Grigious M. Murphy, a gadget salesman who tries to interest them in a personal combat accessory kit.
Colonel K and the houses of parliament have been hit by 'green plus bombs' and have aged 25 years (at least). As Danger Mouse and Penfold close in on Baron Greenback's hideout, they get hit by one green plus and one red minus bomb each, turning Penfold into an old age hamster and Dangermouse a schoolboy detective.
Greenback has invited Dangermouse and Penfold to an old deserted funfair to meet under a flag of truce. Naturally they suspect a trap. Indeed, the villainous toad plans to fire his GB funfair finding, appointment keeping, mouse seeking, Thrissile Missile at the world's greatest detective and his not so very good assistant. However, he did not foresee his pet caterpillar Nero would get too close to the missile and gains super powers.