Set in 17th century London, it serves as a backstory for the original trilogy's main antagonist, Jupiter.

Shortly after his entire family dies of smallpox, young Will Godwin of the town of Adcombe is invited to London by an uncle he has never met.

He and Mr. Balker are told that Will's uncle will meet them at a seedy tavern called the Sickle Moon, but he fails to show up.

While Spittle is away, Will takes the opportunity to explore the upper floor, and finds a room filled with the paraphernalia of a wizard.

Spittle chooses Jupiter, a strong and healthy ginger tabby, whom he trains in the magic arts.

Spittle takes Will and his new familiar to St. Anne's cemetery to call forth the spirit of a wizard from the previous century, Magnus Augustus Zachaire, to aid him in his search for the Philosopher's Stone.

During these talks, Zachaire informs Leech that the latter cannot use any spells because Jupiter has already assumed the role of magic user in his family.

This potion renders the drinker essentially immortal save for two Achilles heels: fire and water.

When Spittle succumbs to the plague and dies, Jupiter reveals his ability to talk to Will and the two finish the alchemist's work on the Elixir of Life to resurrect him.

But when he discovers his sister Dab's preserved corpse, he is filled with rage and drinks the potion himself, turning on Spittle.

Will escapes the burning capital city with Molly, who turns out to have been John Balker's estranged daughter.

Having drunk the Elixir of Life as well as a hair tonic that changes his sable fur to ginger, Leech convinces a rat once kept by Spittle to lead him to the sewers.

Jarvis chose the setting of The Alchymist's Cat from reading the diaries of Samuel Pepys and said he "spent ages revelling in the research.

"[5] Kirkus Reviews called it "a crackerjack creepfest" and added that "Jarvis's florid, purple-tinged prose presents London--with her filthy alleys and crime-ridden alehouses, her overgrown cemeteries and plague-haunted streets--as a major player in the unfolding disaster.

Meanwhile, ghastly revelations pile upon grisly tableaus as the plot hurtles towards a spectacular final conflagration amidst London's Great Fire.

"[6] Christine McGinty of School Library Journal said "The book is filled with adventure, suspense, and a feeling of dread.

"[7] In late 2004, Robin Jarvis announced on his website that a film adaptation of The Alchymist's Cat was to be made the following year.