When a man is found dead near Bishop Grey's home, Alphy and Geordie uncover corruption within the Church's inner circle. Alphy's faith is shaken as he confronts hypocrisy and elitism, while Geordie secures his long-awaited promotion but faces growing distance from Cathy. Meanwhile, Leonard returns to his calling.

As Grantchester swelters in the August heat, Alphy searches for redemption and renewed faith while Geordie investigates the murder of local crook Frank Brook. The case spirals into chaos when armed robbers storm CeCe's Boutique, taking Cathy and Mrs. C hostage. As tensions rise and shots are fired, courage, love, and sacrifice are pushed to their limits.