Villanelle is a Russian orphan who, after murdering the killers of her gangster father, is rescued from prison and trained as a hitwoman by a shadowy group called The Twelve.

[5] Jennings stated that he based Villanelle's character on Idoia López Riaño, a hitwoman for Basque nationalist paramilitary group ETA who was convicted of murdering 23 people in the 1990s.

"[6] Codename Villanelle is a compilation of four serial Kindle edition novellas: "Men or women who are born, as you were, without a conscience or the ability to feel guilt ... without you, without predators—people who can think the unthinkable and act without fear or hesitation—the world stands still.

"[11] Though calling "Jennings' version of 007"—Villanelle—"great fun", Dugdale wrote that the "imperfectly integrated" novella series was repetitious and lacked a "proper denouement".

[5] Similarly, trade magazine Publishers Weekly praised the book as an "exceptional spy thriller" with "superior prose" and "cracker jack plot", noting its "wide-open ending points to more to come in the struggle between these two resourceful antagonists".