Nina and Eddie have been thinking about taking a break to start a family. The IHA has nothing major going on at the moment and Nina has been offered a large advance to write a book-it might be the perfect time for a sabbatical. But those dreams are quickly dashed when the new U.N. liaison calls an emergency meeting to announce that a Viking runestone has been stolen from a Swedish museum, and that the heist was led by none other than a mole operating within the IHA itself.

Nina and Eddie are immediately on the scene, where they learn from the museum's Norse expert, Tova Skilfinger, that the runestone reveals the secret of how to reach Valhalla-a great hall that, according to myth, houses a god-killing poison known as eitr. Using IHA resources on top of Tova's meticulous knowledge, Nina is able to deduce the location of the runestone-it's in Norway-but at the bottom of an isolated lake.

As the team comes closer to excavating the artifact, it becomes increasingly obvious to Nina that Eddie knows more about the mission than he's letting on. In fact, his connection to it is buried deep within his past-in the dense, sweltering jungles of Vietnam-and he knows that to find Valhalla is to unleash a grave danger upon the world. This pulse-pounding adventure will take Nina and Eddie to the vast reaches of the earth in a race against time as they strive to prevent rival forces from unleashing a wave of mass destruction that would threaten the fate of humanity forever.