Black Chamber agents Luz O’Malley and Ciara Whelan are in California, taking a well-earned break after saving the United States from a German plot to unleash horrifying chemical weapons on the entire eastern seaboard. However, the war stops for no one – it isn’t long before President Teddy Roosevelt, gearing up to run for his fourth term, sends them on another mission.

The Germans are rumored to have developed yet another weapon – one that could essentially ensure German naval dominance in the North Sea, which would in turn allow them to continue their march across Europe. It’s up to Luz and Ciara to find out what the weapon is and how to stop it.

They make their way to the continent, only to discover that while they were able to stop the devastation of the U.S., Europe wasn’t so lucky. Some of the great cities – London, Paris – are veritable charnel houses, rendered uninhabitable by the German’s willingness to unleash their “horror-gas” on their enemies. Most of the cities that have survived that onslaught are either being brought to heel by German forces or bursting at the seams from an unending influx of refugees.

Luz and Ciara must go deep undercover in an effort to disrupt the plan before it is too late. Complicating matters is the presence of an agent for the opposition who wants nothing more than to exact his revenge on the pair for the damage they have done both to the German war effort and to him personally. Battle must be waged on scales both large and small – and the fate of the world hangs in the balance.

No one could have doubted after reading “Black Chamber” that there was more to come, but the truth is that often, these sorts of series are subject to the law of diminishing returns. Happily, that isn’t the case with “Theater of Spies,” which keeps the breakneck pacing and exquisite specificity of its predecessor while also allowing for an expanded understanding of our central characters. There’s a sharpness, a clarity to the narrative that invites the reader to devour the book in large chunks.

S.M. Stirling is possessed of an ideal combination of skills for this kind of work. He’s a marvelous tale-spinner, of course, building a plot that is complex without feeling convoluted. He’s got a wonderful sense of character – Luz and Ciara are a delight, both in their personal and professional dynamic (although I will admit that no matter how much I get, I’m always going to want more Teddy Roosevelt). And he’s thorough in his world-building; these things sink or soar on the strength of the details … and we’re definitely soaring with this one.

“Theater of Spies” is smart and compelling, a pulse-pounder of a novel that is pure immersive fun. The bar has been set awfully high for this series – let’s hope that S.M. Stirling can keep it up so that the Black Chamber may continue its mission for years to come.