It's January 1968. Johnny Cash's once successful career is on a downward spiral. He hatches a crazy plan to record a live album at the infamous Folsom Prison in California.

His label, Columbia Records, is totally opposed – his only hope is maverick producer Bob Johnston. Both their careers are on the line.

On the eve of the biggest gig of his life, Johnny discovers a song smuggled out of Folsom Prison, written by inmate Glen Sherley - a violent armed robber. As the concert kicks off, Johnny's decision to perform Glen's song - Greystone Chapel - will change both men's lives forever.

Despite Columbia Records' bleak expectations, At Folsom climbs the charts, as the growing underground scene adopts Cash as a counter-culture icon.

But, this is 1968 – a year of protest, assassination and seismic political change. As the nation slides into turmoil and anger, At Folsom is under threat. Johnny keeps a promise, hatching a plan to get Glen Sherley released.

The success of At Folsom has made Johnny a global star and now he's making good on his side of the bargain - campaigning for prison reform at the US Senate and confronting President Nixon himself. Alongside him is armed-robber-turned-songwriter, Glen Sherley, recently released from prison.

Cash has taken Glen under his wing, setting him on an unlikely path to Nashville stardom.

You can take the man out of prison, but can you take the prison out of the man?

Do the origins of Johnny Cash's mission to save Glen Sherley lie in the reclaimed swamps and dark events of Johnny's Arkansas childhood? As Glen's violent and out of control behaviour threatens the future of the Johnny Cash tour, Johnny has some tough decisions to make.

Probing for the truth behind the rumours, lies and myths, can Danny work out what really happened to destroy Johnny's and Glen's friendship? And what is the legacy of this brilliant but tragic album?