The novel centers on efforts by the main character Court Gentry to track down a mole inside the CIA and later stop a biological attack on an international conference in Scotland.

The Agency-owned transport plane he is in makes a stop at Luxembourg to bring aboard an MI6 rendition team carrying a prisoner, a Dutch banker connected to a mole in the Agency.

Through Brewer, she finds out the circumstances surrounding the death of her father Feodor Zakharov, a former head of Russian military intelligence, in Dagestan fifteen years ago.

Unbeknownst to Zoya, her father had faked his death to become a deep cover agent for the Russian government under the identity of English businessman David Mars.

They later find out about Mars separately; Zoya reunites with her father, who then abducts her for betraying her country, while Court deduces his plan with help from his former handler Sir Donald Fitzroy.

While they dispatch Russian mafia soldiers and some former Spetsnaz mercenaries and shoot down a crop-duster carrying the weaponized bacteria meant for the conference, Mars and Fox manage to escape, with Zoya in pursuit.

Undeterred, Mars storms the conference venue with Fox, his remaining mercenaries, and fellow sleeper agents working amongst the building's security, taking the attendees hostage as a diversion to release the rest of the weaponized plague he had kept for himself.

About the book's tendency to mirror current news, Greaney states, “Mission Critical is a story that definitely takes a lot of things that are going on with the world — North Korea, Russia, U.S. intelligence circles, leaks, all that sort of stuff.