Fantômas introduces not only the book’s eponymous anti-hero, but also his main antagonist, the police detective Inspector Juve. Good and evil, light and dark, these two characters serve to construct the serial’s central duality – a duality which is often blurred, distorted and fractured to the point of psychosis (Juve finds himself copying his enemy’s methodology, and at one point is himself arrested on suspicion of being Fantômas). Other key players who feature in this first installment include: Jérôme Fandor (aka Charles Rambert), the journalist who becomes Juve’s greatest ally in the war against Fantômas – he is also rumoured to be Fantômas’ son, and harbours an unrequited, possibly incestuous passion for Fantômas’ opium addict daughter, Hélène; Lady Beltham, long-time mistress and spellbound accomplice of Fantômas; and Princess Sonia Danidoff, a Russian femme fatale.

Fantômas opens with three heinous crimes: Lady Beltham’s husband, Lord Beltham is abducted and found murdered and stuffed in a trunk; the old Marquise de Langrune is found naked and hacked to pieces at her chateau; and Princess Sonia Danidoff is terrorized and robbed by a masked intruder as she takes a bath in her hotel suite. Inspector Juve blames all three outrages on his nemesis, Fantômas, and sets out to bring him to justice and the guillotine.

Convinced that an English soldier named Gurn – at whose apartment Beltham’s corpse was discovered – is actually Fantômas, Juve pursues him throughout the book and finally manages to have him put on trial for murder.

Gurn is sentenced to death but, aided by Lady Beltham, is able to switch places with an actor, Valgrand, made up to look like him; Valgrand, drugged, is hauled to the scaffold and duly decapitated before Juve can intervene. The book ends with the police inspector’s horror and impotent fury:

"There was a click of the spring, the flash of the falling blade, a spurt of blood, a dull groan from ten thousand mouths, and the head rolled into the basket!

But Juve...thrust the assistants away, and plunging his hands into the blood-soaked hay, seized the severed head by the hair and stared at it... 'It isn't Gurn who has just been put to death!' Juve panted brokenly. 'This face has not gone white – it is painted! It is made up – like an actor. Oh, curse him! Fantômas has escaped! Fantômas has gotten away! He has had some innocent man executed in his stead! I tell you, Fantômas is alive!'"