(Warning - contains spoilers!)
Freddy (Hermione Corfield) has told Baba (Dipo Ola) her darkest secret and enlisted his help for one last time. She doesn't want him to kill anyone, she promises, just teach them a lesson. And Baba believes her.
Together they don Halloween masks to gain entry to her old school, where headteacher Cian Fitzgerald (Kris Marshall) is the man in her sights. As fragments of Freddy's past push back to the surface, she begins to lose her grip on the truth.
In London, Jackson (Babou Ceesay) and Lola (Eve Myles) at last have all the puzzle pieces. Freddy, it turns out, isn't Freddy at all. She was born Lily Jenkins and changed her name in a young offender's institution, where she did time for GBH for an incident at school.
The name she took was the name of her best friend at school. As Jackson and Lola race down to Houndsbury, deep in the Sussex countryside, Freddy challenges Cian to remember what happened to Frederica by the waterside and take responsibility for her death.
As Cian protests his innocence, Freddy's tenuous grip on reality falters.