It's the week before Christmas 1954, and the forecast is for snow in the beautiful Cambridge village of Grantchester. For vicar Sidney Chambers, this is the busiest time of the year and he is rushed off his feet with festive duties. Which is useful, as it means he doesn't have time to think about his beloved Amanda, estranged from her husband and heavily pregnant, who has taken up residence nearby with her Aunt CeCe. Sidney loves her – but whether she's married or divorced, there is no way he can be with her.

Meanwhile curate Leonard Finch is embarking on an ambitious production of the nativity with the local children, and Detective Inspector Geordie Keating is grumpy at the festive spirit - all of which leaves Sidney happy to bury himself in his pastoral work.

Which is when a weeping bride reports the disappearance of her husband-to-be, and Sidney and Geordie find themselves drawn into picking through the murky motives of a winter wedding. When the would-be groom turns up dead with the wedding rings lodged in his mouth, Geordie is horrified - it has all the hallmarks of an unsolved murder from nine years before. Geordie's prime suspect back then was local toyshop owner Albert Tannen. While nothing was ever proved, creepy tales and gossip have turned his once-lively business - and personality - into a twisted shadow of its former self.

Geordie and Sidney must hunt for the killer, and hope to bring justice to two women mourning for their lost loves at Christmas time. But as the dangers grow, snow starts to fall... and waters break...

Sidney Chambers has given up. Following the difficult events of the past few weeks, he is drinking like never before, and even worse, seems to have entirely stopped caring about his responsibilities as a clergyman. With Mrs Maguire and Leonard both deeply worried about him, it is left to them to try and find a way to help their friend before it is too late. But with Geordie and Amanda seemingly unwilling to help, and with an attempt to remove Sidney from the vicarage seemingly gathering strength, things are not looking good.

However, when old friend Reverend Sam Milburn returns to the parish, Sidney is thrown into even greater emotional turmoil. Claiming to be seeking forgiveness for his crimes, Sam's return leaves Sidney torn. Is Sam really here to seek forgiveness? And is it even within Sidney's gift to forgive him for crimes committed against others? Matters are complicated further when Harding and Agatha discover that Sam has returned to the village. With Harding making threats against his life, Sam flees. But when evidence of a struggle is later found, Sidney fears the worst may have happened – could Harding have murdered Sam?

Meanwhile, as Leonard continues to wrestle with his feelings for Daniel, he also struggles to make sense of a life-changing proposal from the Archdeacon. And a visit from Mrs Maguire leads to Amanda having a heart to heart with Guy.

With Sam missing, Sidney and Geordie are forced, once again, to investigate a crime together. However, when they uncover a dark secret about Sam, Sidney's fragile emotional state is pushed even further, putting even greater strain on the investigation. With tensions between the two of them at an all-time high, will they be able to put their differences to one side for long enough to stop a murder being committed?

Sidney is the only friend Gary Bell has left as he waits for news about whether he will be offered a reprieve. Sidney is determined that whatever his fate, he will stand by his side – even if certain members of the village object. Meanwhile, Geordie and Phil visit the hospital where Rita Jones [Tanya Franks] recovers from the latest beating dealt out by her violent husband Eddie [Matthew Jure]. But with her children needing a breadwinner, Rita refuses to press charges. Rita's eldest son lets the two detectives into the hovel they call a family home. When they discover some recently stolen goods, it appears they finally have all they need to collar Eddie. But the man doesn't go quietly, fighting with Phil and winding up Geordie as he's carted to the cells. The next morning, Geordie's wife Cathy calls on Sidney: Geordie didn't come home last night. And when Sidney arrives at the station he finds his friend with his knuckles bruised, and Eddie, dead on a stretcher.

When DCI Benson questions Geordie's innocence, Sidney leaps to his friend's defence. But Geordie has been behaving strangely of late, and no one else can verify his whereabouts when Eddie was killed. At the vicarage, Leonard and Mrs Maguire are at each other's throats as an ill judged remark about Mrs Maguire's love life, and the accidental loss of some sentimental postcards leaves both feeling raw and misunderstood.

As Gary Bell waits to discover his fate, Sidney seeks to avoid a further injustice taking place. But as he digs deeper he finds himself questioning precisely whether he really knows Geordie as well as he thought. Could Geordie have beaten Eddie to death? Will Sidney keep the faith in his friend? And will Gary avoid the hangman's noose?

As the trial of local teen Gary Bell approaches, Sidney and Geordie find themselves at loggerheads regarding his guilt – not even building a go-kart together can get them to see eye to eye. Sidney's mind is taken off these matters when he finds the anxious Reggie Lawson [Paul Nicholls] at the graveside of his dead wife, Anna. Convinced that she still haunts his home and stable yards following her suicide he implores Sidney to come to his estate, Kingsbrook and perform an exorcism for him.

Believing this to be a husband's desperate grief, Sidney goes with Reggie to investigate his claims. There he meets Reggie's new wife Kitty [Eliza Hope Bennett], their housekeeper Mrs. Elton [Jenny Ogilvie] and Anna's brother, Laszlo Herzl [Elliot Levey]. With Anna's signature perfume still hanging in the air, a painting falling from the wall of its own accord and a horse fatally kicking out after being spooked, Reggie's fears seem to have some foundation, although Kitty is quick to point out their coincidental nature. After offering prayers and council, Sidney leaves hoping that his words have given Reggie some solace. But that night, Geordie summons Sidney back to Kingsbrook: Reggie has hanged himself in the exact same spot as Anna died.

Meanwhile, the Redmond's and Sidney's parishoners are unhappy when Sidney stands as a witness for the defence in Gary's case. Are his principles going to cost him his flock? With Sidney's sister Jen [Fiona Button] throwing a party in London, it's a chance for Sidney to escape and take Margaret out on his arm. But when Amanda shows up with Guy [Tom Austen], it's clear that his own feelings are still conflicted.

At least investigating Reggie's death gives Sidney and Geordie a shared goal again. Why would Reggie take his life when his finest horse is set to triumph at the next race? And with an almost identical suicide to his late wife, why are some of the key details different? But is one shared case enough to get these best friends back on track - especially after Sidney takes the stand? Whatever the outcome of the trial - and the case - Sidney seems set on a course of action that threatens his friendship with Geordie...

After thoroughly modern Margaret's visits to the vicarage start leaving Mrs Maguire's nose out of joint, Sidney is hoping for a distraction – which arrives in the unexpected form of a young man sat alone in the church and covered in blood. The man is Theo Graham [Jeremy Newmark Jones], and he confesses to Sidney and Geordie that he attacked and killed his landlord, Eric Whittaker [Michael Shaeffer]. But when Sidney and Geordie call on the Boarding House that Eric ran, they find the alleged victim alive and well, along with his teenage daughter Joan [Rosie Day] and her step-mother Vivian [Liz White]. Eric believes Theo is a fantasist. Back at the station, despite his protestations that Eric will come after him, Geordie sends the young man on his way. Hours later, however, witnessed by Sidney and Geordie, Eric is killed in a hit and run.

Sidney comforts Vivian at the hospital, but back at the boarding house he finds her relationship with Joan to be strained – all the more so when Joan learns of her father's death. When Theo arrives to claim his belongings, Sidney ends up chasing him from the boarding house, but the pursuit ends when Theo slashes Sidney with a knife.

As Vivian tends to Sidney's wounds, he becomes drawn deeper and deeper into the twisted emotional world of the boarding house. Who is Vivian trying to protect? What does seemingly shy and single boarder Raymond [John Voce] really want? And how far will Sidney have to go to uncover the killer.

With Leonard distracted by secretive trips to the cinema and Geordie compelled to deal with a surprising shoplifter, who is watching Sidney's back?

Geordie and Cathy [Kacey Ainsworth] are on a mission to find Sidney a girlfriend – but nobody seems to be good enough for the choosy vicar. Geordie's plans are further thwarted when Sidney's old flame, Amanda Hopkins [Morven Christie] arrives in Grantchester while Guy is away on business.

Geordie calls on Sidney for help following an apparent suicide at Sidney's old Cambridge College. Valentine Lyall, a fellow at Corpus Christi, has been found dead: the result of a long fall from the spire of King's College chapel. His wife, Mya [Lourdes Faberes], is consoled by Valentine's star pupil, Kit Bartlett [Matthew Tennyson] who used to partake in daredevil climbs of the college buildings alongside his mentor. But both are convinced that Valentine was too happy and content for it to be suicide.

Kit's alibi is Russian Tutor, Professor Raban [Tim McMullan], who is mentoring a chippy student and wannabe Marxist called Rory Crompton [Josh Bolt] when Sidney and Geordie arrive to question him. They receive an even cooler welcome from Master Giles Montgomery [Nigel Planer] who remembers Sidney from his student days – and still resents him for an unmentioned past transgression.

It appears to be a case of a night climbing misadventure until they discover a student witness who saw someone push Lyall to his death. Hastily burnt papers and unexplained bloody handkerchiefs lead Sidney and Geordie deeper and deeper into a web of espionage and secrets. Who is the mysterious woman watching them? Why is Geordie so unnerved by this case? And will police secretary Margaret Ward [Seline Hizli] finally be the woman to break Sidney's run of bad dates?

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?