It's 1954, and just outside the Cambridgeshire village of Grantchester, local vicar Sidney Chambers is enjoying a glorious picnic with his best friend, DI Geordie Keating, Geordie's family, Sidney's shy curate Leonard Finch and their grumpy housekeeper Mrs Maguire. Walking home, their happy day is cut short when DC Phil Wilkinson arrests Sidney – and the charge is sexual assault.
Doting father Harding Redmond is the source of the accusation, horrified at the claims made in the diary of his teenage daughter, Abigail [Gracie Brooke], who has now gone missing, along with the diary itself. After being grilled by DCI Benson, Sidney sets out to prove his innocence, but not before the Archdeacon sends Sidney's old friend Reverend Sam Milburn [Andrew Knott] over from the neighbouring parish to check up on him.
Questioning Abigail's friends at the village youth club, Sidney discovers she had recently spent time with local photographer Daniel Marlowe. Sidney and Geordie discover Daniel's studio is empty – except for the lifeless body of Abigail.
With Daniel missing, Sidney and Geordie talk to local teenager Gary Bell, who had previously been accused by Abigail's father of inappropriate behaviour toward her, as well as her family and friends. However, as more and more secrets begin to surface about Abigail's life, it becomes clear that no one knew her as well as they thought. Only by finding her missing diary can Sidney and Geordie pinpoint exactly who was responsible for her death – but will the answer damage their friendship forever?