Marcella tries to make sense of the previous evening's events. Meanwhile, the Gibson family are in a state of panic when Grace is discovered missing, and Marcella scours CCTV footage in an attempt to discover more about Grace's disappearance. Cara returns home from her latest Sinnr exploit. She's managed to swipe a watch, some earrings and a bracelet. Cara's unaware of their significance and the danger she's put herself in - all of these items are recognisable as trophies taken by the Grove Park Killer. Peter Cullen continues his prison work placement at the bakery and is delighted by his budding friendship with a young postgraduate student called Maddy. Marcella warns Maddy that Peter is dangerous, but Maddy assumes she's safe - Peter's supervisor Guy is always with him. But later, we see Guy head out with Peter and leave him to his own devices.

Clive Bonn is brought in for questioning, but his immoral motives for being at Hampstead Heath near Grace's burial site exonerate him from being the murderer. Maddy can't get hold of Adrian after their last fight and Peter is still pursuing her. He surprises her on her doorstep and walks her to her university. As she's about to leave, he gives her a gift-wrapped present, its significance will later be horrifically telling for Maddy. Laura and Rav question Marcella about why she was looking at CCTV from Grace's house, but Marcella manages to conceal the truth. She's told to look through it again for a minicab as one has been seen near two of the previous victims' houses. Laura pressures Marcella to give her DNA after blood is found at Grace Gibson's house. Marcella, realising that the blood is probably hers, devises a plan to avoid the truth coming out. Marcella's brilliant ability to deceive once again comes to the fore.

Marcella is back to the drawing board after realising Peter Cullen can't be the culprit for the current spate of murders. Cara's sex-cam threats lead back to the IT business owned by the Gibson’s family friend, Matthew. When questioned he denies having any involvement. Marcella plays Peter at his own psychological games in the interrogation room, taunting him as a disappointing failure. His pride pricked, Peter cracks and makes a confession. Jason meets with DTG's intelligence firm to get leverage on Andrew Barnes so he can claw his job back by getting the Lambeth project back on track. The firm's agent, Stuart, explains that Barnes is spotless. Afterwards, in private, Jason accepts some "unofficial" help from Stuart in dealing with the man blocking the development. Their conversation is left open ended and open to much interpretation.

Marcella and the team race against the clock to find six year old May, whom the killer has abducted. The suspicion is that May witnessed her babysitter, Beth being attacked and startled the killer. The police start tracking May's location, but are soon thwarted much to Marcella's despair. The SOCO forensic officers find strands of hair in Beth's lifeless hand, but tests for DNA will take a couple of days. A neighbour claims he saw someone in a blue hoodie carrying the girl - which CCTV footage confirms. Marcella is on a very personal quest, trying to get into the mind of the killer: why not kill May on the spot? Can they get to her before he makes any more decisions? Marcella locates the killer's burning car in the wastelands surrounding City airport. May is lying face down in a puddle, hands and feet bound by cable ties. Marcella tries desperately to resuscitate her, but is it too late?