It's 1955 in Grantchester, local vicar Sidney Chambers [James Norton] meets with the new archdeacon, Gabriel Atubo. Acknowledging that Gabriel's predecessor was dispatched under some controversy and embarrassment, Gabriel questions whether he can rely on Sidney to rebuild trust in the church and lead by example. Sidney reassures him he can be counted on.

A quiet day in Cambridge is thrown into disarray as Leonard and his new lady friend Hilary are witness to an armed robbery at the post office. With a gun pointed in his face, Leonard attempts to talk down the masked culprit and receives a strike to the head for his bravery.

Back from a morning run, Sidney is joined by former flame Amanda at the vicarage. Having agreed not to take their relationship forward, they exchange pleasantries but keep their distance. As Amanda leaves, Mrs Maguire encourages Sidney to stay strong. Sidney's longing for Amanda only worsens when he later sees her with estranged husband, Guy. Could Amanda be contemplating a return to her old life?

As Leonard is hailed a hero, Sidney and Geordie investigate the post office robbery and it soon becomes clear, this is the first of many. As the robberies become more violent, Sidney and Geordie must work quickly to unmask Cambridgeshire's very own Bonnie and Clyde.