Fifteen-year-old Mark Davis never expected to become a detective. A sophomore at Westfield Preparatory Academy, Mark lives with his widowed mother and his grandfather, a Vietnam veteran struggling with dementia. His ordinary life is shattered when a local rabbi is murdered in a nationally publicized hate crime, and the killer disappears without a trace.

As media attention engulfs his hometown and law enforcement fails to locate the fugitive, Mark becomes obsessed with the case. Determined to help bring justice to the victim's family, he begins conducting his own investigation from his bedroom computer. What starts as curiosity soon turns into a startling discovery when Mark uncovers a possible connection between the wanted killer and a beachfront vacation property.

Convincing his unsuspecting mother to book a family getaway at the condo, Mark finds himself vacationing in what may be the fugitive's personal hideout. As he searches for clues, every discovery pulls him deeper into a dangerous mystery involving hidden identities, secret relationships, and a man willing to kill to protect himself.

Armed with little more than determination, faith, and an eye for details others overlook, Mark must decide how far he's willing to go in pursuit of justice. Along the way, he learns that courage is not the absence of fear, wisdom often comes from unexpected places, and even someone who feels ordinary can become remarkably useful.

A fast-paced Christian mystery filled with suspense, humor, and heart, Useful Mark is an engaging coming-of-age detective story about faith, family, and the pursuit of truth.