The discovery of Catriona Bruce's body leads DI Perez to take Magnus Bain into custody for further questioning. However, Magnus becomes deeply distressed at the station and attacks Perez. While they await the results of psychological tests on Magnus, Perez re-examines the evidence from the Catherine Ross case. Informed that Catherine used her phone mostly to create portraits, Perez is desperate to find any photos that she may have downloaded onto her laptop. He is frustrated, however, to discover that all evidence has been wiped. Viewing CCTV footage of Catherine clutching her bag at the bus station, Perez wonders if its contents hide a secret.

When the post mortem results verify Cora's theory about Catriona's death, Perez reports back to Catriona's brother, who subsequently reveals that Catriona had a secret hiding place at their old home (where Catherine had lived). After digging through old police reports, Perez gleans why Magnus hates being in the police station and moves him to a place where he can feel safe. Magnus finally trusts Perez enough to reveal exactly what happened on the day Catriona disappeared: how the little girl saw something that greatly upset her -- something that involved Margaret Henry. Magnus also reveals that Catherine knew Catriona had a hiding place...with another girl in imminent danger, the team finally find the last, vital, clue which leads them to the killer -- and the biggest twist of all.