The legendary Independent Agent is going into retirement so he invites 6 of the world's best agents (well 5 of the world's best agents and his grandson, who is a corporate spy) to compete in a contest. The group is to solve 5 mysteries, and the prize for the winner (there can be only one) is the Independent Agent's vast horde of secrets, including the name of the traitor inside the Drood family.
I believe three of the other competitors are new characters, but readers of Green's Nightside series will certainly recognize Walker, and the Blue Fairy is from previous books in this series.
The mysteries are quite interesting, ranging from discovering the truth about the Loch Ness monster to the Philadelphia Experiment. They're each interesting mysteries, although my one quibble is that Sasquatch is from the American Pacific Northwest, not Arkansas. I'm sure Arkansas has some lovely legends and myths of its own, but as a native of the Pacific Northwest, I feel compelled to point out that Sasquatch belongs to the Pacific Northwest.
Predictably, since there can be only one winner, the agents begin being murdered, one by one. There's never any real concern that Walker would die, he's too major a character in the Nightside series to die in a different series, and, of course, Eddie Drood has to survive, but the rest are fair game.
I didn't see the twist at the end but once it was revealed it was blindingly obvious.
I really enjoyed learning more about Walker and seeing him when he's not interacting with John Taylor. It was also nice to see Eddie away from the politics of the Drood family and his girlfriend and just be himself on a mission. I like Molly, but I'd like to see more of that.