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.

Beth tries to connect with her son, Arthur, who has built a metal detector with his friends to help them investigate the mysteries in Granite Flats. A big city reporter begins asking questions around town.

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.

Chief Sanders and Agent Scott investigate citizens whose loyalties are questionable, the young detectives are still curious about the coded message they found, and at the hospital, experiments reveal a patient's past.

The three sleuths argue about getting involved with the FBI investigation. Beth is concerned about Frank and the hospital experiments. Wallace is caught in the middle when Regina and Hershel argue about what's best for him.

Timmy, Arthur, and Madeline make a dangerous discovery, right under the nose of the FBI. While they investigate, Granite Flats is in an uproar - chaos breaks out at the hospital and Chief Sanders races to apprehend a suspect.

Everyone tries to deal with the loss of a friend, and sorting it out reveals vulnerabilities: Regina keeps reflecting on better days, John can't seem to live with himself, and even Hershel struggles to deal with the tragedy.

The kids' new English teacher has a way of making Hamlet applicable to life in Granite Flats. Some citizens are unsettled when a CIA agent arrives. And Arthur's detective work reaches a new level.

Ashmead stirs up confusion and danger when he meddles in the Andrews' personal and professional lives, and Chief Sanders recruits a new deputy with somewhat surprising results.

Arthur continues his investigation alone and finds a resonant soul in Whittison. Hargraves counsels Timmy on romance. And Hershel becomes the bait in a dangerous trap.

In the dramatic season finale, crisis is the watchword. Beth, Whittison, and the Andrews turn the tables on Ashmead. Timmy and Madeline find what everyone has been pursuing. And Professor Hargraves may not be what he seems.

A couple of unexpected visitors come to town. The kids are keeping secrets. Professor Hargraves welcomes someone back to class. John looks for work. Hershel gets a special delivery. The FBI and the CIA search for The Plumber.

The Plumber investigation takes a step forward. Madeline makes a new friend. Regina gets a surprise. Trust issues arise for the Andrews. Scottie takes a case. An old secret comes to light.

Unexpected visitors interrupt church. Sunday dinner doesn't go exactly as planned. John commits a crime. An old friend spends the day with Frank.

A new suspect emerges. Susan studies Alice. Frank and Whittison find common ground. The Sanders take a long drive. Wallace gets the shock of his young life.

Alice goes on a date. The Andrews investigate a possible KGB mole. Ezekiel and John get tough. Hershel avoids confrontation. June has an unexpected dinner companion. Things are bumpy for the detectives.

Ezekiel works in disguise. Hershel faces some truths. Susan seeks comfort in a dangerous place. Hargraves gets a strange call. John has a suspect.

Hargraves makes a bold move. John doesn’t follow the rules. Wallace’s fate is uncertain. Alice is asked to interpret her vision. The kids turn to a surprising ally. Frank takes a trip.

Regina has a late visitor. John and Hershel confront realities. Timmy and Madeline knock on the wrong door. Wallace learns of his future. Whittison faces his past.