The Three Dog Detective Agency has always worked to protect the weak, ensure justice for the oppressed, and occasionally save the world, but those efforts have not always been appreciated by Officers Antony and Arnold of the Chula Vista Police Department's crack K-9 Unit. Those two respected and highly decorated police dogs often find themselves at odds with Levi, Sunny and Yoda, the three dogs who operate the detective agency. In previous adventures, the police dogs were willing to look the other way when the Three Dog Detective Agency protected its clients and pursued its own cases, at least in the end, because malefactors were brought to justice and credit was given to the K-9 Unit. In this latest adventure, however, Officers Antony and Arnold find themselves going head-to-head with Levi, whom they believe is hiding a Whippet named Slim Shady, an accused thief and biter. Unknown to either the K-9 Unit or the Three Dog Detective Agency, Slim Shady is but a pawn in a much larger game. An ancient evil rises from the shadows, and those touched by its paw are changed forever. Officers Antony and Arnold find themselves disgraced, accused of heinous crimes, and dismissed from the K-9 Unit. Humiliated and humbled, they must seek help from dogs they have often held in disdain...only to discover the Three Dog Detective Agency is already on the job. Once again, the Three Dog Detective Agency must battle evil and injustice, but now their enemies strike from the shadows and nothing is as it seems. This time evil wears a mask.