First-time novelist Matthew Dunn brings an impressive background to the world of espionage fiction: A veteran field operative for MI6, Britain’s intelligence service, he ran covert operations around the globe, boasting 70 successful missions. Surely such behind-the-scenes expertise would translate into grippingly realistic fiction, right?

At times, “Spycatcher” delivers just that: terse conversations infused with subtle power plays, brutal encounters among allies with competing agendas, and forays into hostile territory orchestrated for clockwork efficiency but vulnerable to deadly missteps.

When “Spycatcher” opens, messages intercepted by the National Security Agency reveal an imminent assault against Western interests. A joint endeavor between the CIA and MI6 pits British agent Will Cochrane against the plan’s mastermind, the shadowy Megiddo, a top-ranked officer in Iran’s Revolutionary Guard. Cochrane sets out to lure Megiddo into the open or else be captured himself and likely tortured — whatever it takes to get closer to his prey. Cat-and-mouse games ensue, with no certainty as to who’s playing whom. Cochrane’s chief asset is a Paris-based journalist who was Megiddo’s lover during the Bosnian War. Cochrane is also joined by a quartet of American operatives whose collective résumé includes stints with the Navy SEALs, the Green Berets and Delta Force — providing the jaws of a ferocious mousetrap.

Cochrane’s past haunts and complicates the unfolding action — particularly the murder of his father, himself a British agent. He also risks being compromised by a budding appreciation for that Paris-based journalist. Is it fair to use her as bait? To sacrifice her for the greater good? Reluctance and regret imbue nearly every action.

But while building this compelling storyline, Dunn falls into some unnecessary exaggeration. Not just a special agent, Cochrane has to be a super agent — the sole member of a top-secret Spartan program. As one handler tells him: “You are the ultimate killer of killers, the man who terrifies his enemies and allies, the man who can start wars and end them, the man who is the West’s deadliest and most secret weapon.” Similarly hyperbolic, Megiddo’s plot promises “a huge massacre the likes of which the world has never seen before,” and Megiddo himself is cast as some dark overlord: “Not one major terror act against Western or Western-allied targets can take place without his implicit or explicit authorization. Even groups that are the sworn enemy of the regime of Iran find themselves working for him, usually without knowing they’re doing so.”

That unevenness — stark realism meets cartoonish excess, male fantasy mars persuasive credibility — undermines what otherwise stands as a stylish and assured debut.

Taylor frequently reviews mysteries and thrillers for The Post.

SPYCATCHER

By Matthew Dunn

Morrow. 418 pp. $25.99