1962: Granite Flats, Colorado. A quiet, friendly little place...until part of the army base explodes. Is it a Soviet attack or an inside job? Who are the mysterious men in dark suits driving around town? What clandestine work is being done at the local hospital? As three young amateur sleuths and police chief investigate these mysteries, they begin to unravel a complex web of secrets, and Granite Flats will never be the same.

The base explosion causes a Red Scare, and Arthur and his new friends begin investigating those who may have been involved with the explosion.

Arthur and his friends Tim and Madeline look into the town's mysteries, and the police chief investigates one man's part in the base explosion.

Friendships are tested after a failed class project, Beth investigates Frank's strange behavior, and John struggles to balance his family and his duties as police chief.

Arthur and his friends look for more mysteries to solve. John, the police chief, works with the FBI.

Frank asks for Beth's help to prove his sanity. Hershel receives visitors while incarcerated.

John races to free Hershel from prison, and the kids uncover real danger after the science fair.