By Clea Simon

When The Secret Hours flares up ­­– notably on two separate, devastating occasions ­­­– the story delivers more emotional heft than Mick Herron’s previous books.

The Secret Hours by Mick Herron. Penguin Random House, 384 pp., $27.95.

Fans of Mick Herron’s “Slough House” books have come to expect a certain standard from the UK thriller author. There will be clashes with bureaucratic higher-ups, political coverups, and various colorful (and often hilarious) fuck-ups of every variety. The most vibrant of the latter will consistently be Jackson Lamb, the overseer of the misfits who staff Slough House (nicknamed the “Slow Horses”), a kind of dumping post for British Secret Service rejects and failures who can’t be easily sacked. Proudly flatulent, Lamb makes a statement with his open disdain for authority as well as personal hygiene and the Service throughout the eight-book series. His one loyalty is to his “joes,” the agents who risk it all in the field.

In contrast, Herron’s new standalone, The Secret Hours, at first appears to feature competent, if retired, intelligence officers. Out in the countryside, purported retired academic Max Janacek is enjoying his morning walk when he realizes he’s being followed. When the professed professor expertly wards off an attack, he immediately connects the bungled attempt with his true past, as a Cold War spy, and realizes that, to prevent further attempts on his life, he must track down someone who leaked after 50 years of silence. Meanwhile, back in London, a government committee has been formed to poke into Service business. Monochrome, its purposely bland name, is theoretically looking into spending issues, but the unnamed, politically astute First Desk knows full well that it’s the spear tip of a power play by the Prime Minister, seeking to control the fiercely independent agency. The usual political-bureaucratic wrangling seems to be almost done ­­– the Service rarely loses such battles ­­– when an old file reaches Monochrome, one that threatens to blow the Service apart.

From that point on, the book’s timelines split. In the present, we have Max doing his best to trace the source of the leak — and to fend off more attacks. We have the political fencing, with the cold-blooded First Chair facing off against the PM’s right-hand man. Then we have the Monochrome inquiry, where witness #137 takes us back to her days as a newbie in Berlin, immediately after the fall of the Wall. Despite her lack of experience or training, that newcomer, then known as Alison North, has been assigned to look into irregularities — and to report back directly to David Cartwright, the Service’s powerful second-in-command. She’s soon sussed out by the Berlin chief and finds herself weighing dueling loyalties, trying to make sense out of a conflict that she had thought as worthless as the lumps of concrete being drilled apart everywhere to be hawked as pieces of the Wall.

That this case will serve as a backstory for Cartwright, Lamb, and another beloved character from the “Slow Horses” books is lagniappe. It’s a great back yarn on its own — revelatory about the Service structure and its leaders. But this novel offers more than that.

“Slow Horses” fans should be aware: This narrative starts out much more deliberately than its predecessors and with a tad less of the author’s customary humor. Indeed, much of The Secret Hours trundles along at a slow burn, almost more like a police procedural than an espionage thriller. The writing remains superb, however, and each careful step is worth following. Because when the story flares up ­­– notably on two separate, devastating occasions ­­­– this volume delivers more emotional heft than Herron’s previous books. The first revelation is subtle — a quiet insight that leads to a confession. The second, which apparently ends an unhappy work relationship while igniting a romantic one, is delivered with startling understatement, a silence that changes the world. The Secret Hours isn’t exactly a departure from Herron’s earlier books. Nor is it really a standalone. It’s simply a masterful new book from today’s premier espionage author.

Somerville-based novelist Clea Simon is the author most recently of the psychological suspense Hold Me Down. She can be reached at www.CleaSimon.com.