Karin Schaeffer and Mac MacLeary have settled into their lives in Brooklyn where they share a brownstone with their five-year-old son Ben and newly adopted thirteen-year-old daughter Dathi.
In what was seems like a routine job, Mac is hired by Cathy Millerhausen to investigate her billionaire husband's suspected infidelity. But by the look of it, Godfrey Millerhausen is faithful to his wife: the case is a dud. So when Mac is offered a high-paying "quickie" assignment in London, he wraps up the Millerhausen case to take what evolves into a family vacation culminating in a home exchange on the Mediterranean island of Sardinia, Italy along with their friend and assistant Mary Salter, and her teenage son Fremont.
Karin and Mac spend a few days together in England while Mary travels ahead to Italy with the three children. After completing the London job, Karin and Mac arrive at the Sardinia house where they find their family's belongings, but no trace of Mary or the kids.
A missing persons investigation is launched by the local police, whose low-key approach to law enforcement unnerves Karin and Mac. They decide to conduct their own investigation. When it occurs to them that the seemingly innocuous Millerhausen case could have something to do with their missing family, Mac returns home while Karin stays in Sardinia to keep searching.
With the help of his friend Detective Billy Staples, Mac follows the money into some surprising corners of Godfrey Millerhausen's life, barely surviving an elevator explosion when he gets too close to the truth. Back in Sardinia, Karin is nearly run off a cliffside road in a harrowing high-speed car chase. It becomes clear that someone is going to great lengths to try to stop Karin and Mac from unearthing details about the elusive billionaire.
By the end, Godfrey's misdeeds are darker than anyone imagined, involving an international conspiracy that summons law enforcement from around the globe.
When Karin and Mac return home with the family, they face a harsh new set of realities that both expands their family and challenges their faith in their own instincts. In a final blow, they learn that the real perpetrator of the crimes is someone they never suspected.