The Crossing Places

Dr. Ruth Galloway, a forensic archaeologist, and Detective Chief Inspector Harry Nelson cross the Norfolk Saltmarsh at dawn. They are heading to a prehistoric henge circle, where Ruth kneels and begins to dig in the mud. She uncovers the tiny arm of a child wearing a christening bracelet, confirming a dreadful suspicion.

The narrative flashes back to introduce Ruth, a lecturer at the University of North Norfolk who lives alone in an isolated cottage on the marsh. At work, her department head, Phil, introduces her to DCI Nelson. Nelson explains that a child's bones have been found on the Saltmarsh, and he needs her to determine their age. Ruth accompanies him to the site near a bird sanctuary. During the excavation, she finds a hand wearing a grass bracelet and a metal object she identifies as an Iron Age torque, a type of necklace. This suggests the bones are ancient, not modern.

On the drive back, a disappointed Nelson confides that he hoped the bones belonged to Lucy Downey, a five-year-old girl who vanished from the area ten years earlier. He reveals that during that investigation, he received anonymous letters claiming Lucy was a "sacrifice" and would be found "where the earth meets the sky." Nelson reflects on the cold case and is intrigued by Ruth’s professionalism. Back at her cottage, Ruth examines the torque and recalls a henge dig she participated in on the Saltmarsh ten years prior, led by her mentor, Erik Anderssen. The next day, she returns to the site alone and finds a second torque. As a storm rolls in, she becomes lost on the marsh in the rising tide. She is rescued by her neighbor, David, the warden of the bird sanctuary, who navigates a hidden path marked by sunken wooden posts. Realizing this could be a prehistoric causeway, Ruth calls Erik in Norway with the news. Interspersed with these events are brief scenes from the perspective of a young girl held captive in a dark, stone-walled room.

After Christmas, Nelson summons Ruth to the police station. He informs her that another child, four-year-old Scarlet Henderson, has disappeared from the nearby village of Spenwell, almost exactly ten years after Lucy Downey. Nelson has received a new letter, seemingly from the same author, and asks Ruth to analyze all of them. At home, Ruth categorizes the letters' dense allusions to the Bible, Shakespeare, Norse mythology, and archaeology, including the technical term "cursuses." Suspecting a connection to the henge dig, she gives Nelson the name of the druid leader from that time, a man known as Cathbad. Soon after, Erik calls to say he is coming to Norfolk for a sabbatical.

Later, Nelson asks Ruth to accompany him to question Michael Malone, also known as Cathbad. They find him living in a caravan and learn he was one of Erik's archaeology students and now works as a lab assistant at the university. The investigation takes a sinister turn when Ruth finds her cat, Sparky, killed on her doorstep, an act Nelson interprets as a threat connected to the case. Nelson comes to collect the body for forensics, warning Ruth to be careful. That same night, Erik arrives at her cottage unexpectedly. The next day, while they are on the marsh examining the causeway posts, Nelson calls, human bones have been found in Scarlet Henderson's garden. Ruth goes to the site and determines these bones are also Iron Age. In the Hendersons' kitchen, she sees a photograph of Scarlet's parents, Delilah and Alan, with Cathbad.

Ruth informs Nelson about the photograph, and he soon discovers that Cathbad is the father of Delilah's eldest daughter. Meanwhile, Ruth's ex-boyfriend, Peter, returns to the area. While walking on the marsh, Ruth and Peter encounter David near a remote bird-watching hide. The case appears to break when Nelson texts Ruth that Cathbad has been arrested, his fingerprints were found on the letters. Erik is furious, accusing Ruth of helping Nelson frame an innocent man. Poring over her maps, Ruth realizes the Iron Age bodies and the causeway form a straight line pointing to the henge. Acting on her theory, police search the henge's center and find Scarlet's body, preserved in the peat. That night, a devastated Nelson comes to Ruth's cottage after informing Scarlet's parents. He reveals another letter has arrived, sent while Cathbad was in custody. Overwhelmed by the day's events, Ruth and Nelson sleep together.

To escape the press, Ruth stays with her friend and colleague, Shona, where she receives an anonymous text: "I know where you are." Nelson calls to inform her that Cathbad is being released, as forensic evidence does not link him to Scarlet's murder. After Scarlet's funeral, Ruth returns home and is confronted by Cathbad. He asks her to clear his name and reveals that Shona and Erik had an affair during the henge dig. Ruth confronts Shona, who admits to the affair and to helping Erik write the anonymous letters. She explains Erik despises Nelson, believing he framed one of his students, James Agar, for the murder of a policeman years ago in Manchester. Agar later committed suicide in prison. The letters were intended to distract and taunt Nelson. Stunned, Ruth realizes the handwriting in the letters matches a dedication Erik wrote in one of his books. She goes straight to the police.

Nelson's team moves to arrest Erik, but he has vanished from his guesthouse. Ruth returns to her cottage to pack for a trip away and is confronted by Erik. He denies the murders, but Ruth flees onto the Saltmarsh during a violent storm, with Erik in pursuit. Erik calls Cathbad to say he is going to see Ruth, prompting Cathbad to alert Nelson, and they race to the marsh. Lost and terrified, Ruth stumbles upon the remote bird-watching hide. She hears cries for help from a trapdoor in the floor and discovers a cellar where a teenage girl is imprisoned. Ruth realizes it is Lucy Downey. As she helps Lucy escape, they are confronted by David. Ruth understands he is the kidnapper and murderer. David confesses he accidentally killed Scarlet while trying to abduct her as a new companion for the maturing Lucy. He also admits to killing Ruth's cat.

As David pursues them with a knife, Lucy emits the distinct call of a Long Eared Owl, a bird David had been protecting. Lured by the sound, David walks into a deep tidal pool just as a large wave crashes in, sweeping him away to his death. Nelson arrives to find Ruth safe with the rescued Lucy Downey.

Three weeks later, Ruth reflects on the aftermath. David's body was never found, while Erik's was discovered drowned in a marsh pool near her cottage. Lucy is reunited with her parents and is slowly recovering. Cathbad is cleared and has formed an unlikely friendship with Nelson. Ruth meets Nelson on the beach to discuss Lucy's progress. The mystery of the Iron Age girl remains unsolved. As they walk by the sea, Ruth keeps a secret of her own, she is pregnant with Nelson's child.