Who Lisa Treherne is
Lisa Treherne is Cecily’s sister and a member of the family that runs Branlow Hall. She is less welcoming than her parents when Susan arrives to investigate Cecily’s disappearance. Her scepticism gives the hotel an immediately unsettled atmosphere: the family has paid for help without agreeing about the person providing it.
Role in Moonflower Murders
Lisa challenges the idea that reading a novel can justify Susan’s fee. Her hostility also directs attention towards older tensions between the sisters and towards the hotel’s employment arrangements. The story asks the audience to distinguish a difficult, sometimes cruel personality from proof of a particular crime. Lisa can be wrong about Susan’s usefulness without being wrong that the investigation will disturb the family.
Key relationships
Lawrence Treherne and Pauline Treherne are her parents. Aiden MacNeil is her brother-in-law. Stefan Leonida and Liam connect her to a less flattering account of life among Branlow Hall’s staff. Her fictional counterpart is Melissa James.
Played by Rosalie Craig
Rosalie Craig plays Lisa Treherne. Craig’s two characters occupy very different positions in their investigations. Lisa answers questions in the contemporary story; Melissa’s death creates the central question inside the book.
Their story and fate — spoilers
The investigation reveals Lisa’s exploitative relationships with employees and her hostility towards Stefan. Those facts complicate the conviction for Frank’s murder, but Aiden is the killer. Susan also corrects an assumption about the couple seen in the woods: Stefan was with Cecily, not Lisa, and Roxana is his daughter. A witness’s confidence about Lisa had concealed a different relationship.
Books and television — book spoilers
Rosalie Craig plays both Lisa and Melissa. Conway’s fictional transformation turns a hotel-family member into a former film star and murder victim. The contrast is deliberately unfair: recognising Lisa in Melissa does not make the actress’s invented biography a faithful account of Lisa’s life. Television can make the resemblance immediate through casting, while Susan still has to determine which parts of the portrait contain a useful clue.
Role in the story
Lisa makes Susan's reception at Branlow Hall noticeably awkward. Her parents have invited the editor, but their decision has not created agreement within the household. Lisa challenges the value of the investigation and has complaints of her own about the arrangements that favour Cecily and Aiden. For a viewer arriving midway through the series, she is the sister whose dissatisfaction makes the hotel's polite welcome feel incomplete.
Her role also draws attention to the people who work for the family. The difference between a hotel's public hospitality and its private employment relationships becomes increasingly important. Susan cannot assume that the account offered by an owner carries the same meaning for a vulnerable employee. Lisa's forceful manner makes those imbalances easier to notice, although an unpleasant encounter is not a solution to the murder.
Key scenes by episode
Follow the linked episodes as you watch. Open the scene notes when you want more detail.
Episode scenes (full spoilers)
Episode 2: At lunch, Lisa questions Susan’s fee and complains about the benefits enjoyed by Cecily and Aiden, including the cottage. Lawrence later explains the childhood incident behind Lisa’s scar. The episode places an old grievance beside current resentment without treating them as the same event.
Episode 3: Lisa and Liam reveal their ability to threaten one another. Their interaction gives Susan another reason to look beyond the hotel’s official version of its staff relationships.
Episode 5: Liam describes his relationship with Lisa and says he saw Stefan with her in the woods. His statement sounds specific, but Susan still needs to test whom he actually saw. Later Lisa tells Susan to leave after the news about Cecily.
Episode 6: Stefan’s account changes Susan’s understanding of the pressure he faced. The supposed woodland meeting with Lisa is also corrected: the woman was Cecily. A confident identification has helped conceal a relationship with consequences for the whole family.
In the book
Book and screen comparison (book spoilers)
Melissa James is the character to compare with Lisa inside Conway's novel. The hotel owner's daughter becomes a former film star and murder victim in his fictional treatment. Rosalie Craig's dual casting makes the connection clear, but Melissa's career and experiences cannot simply be transferred back to Lisa. Read the two introductions together before opening either ending, then compare how each woman's position affects the people who depend on her.
What to notice on a rewatch (spoilers)
Lisa's scenes are especially useful on a rewatch when somebody else describes her actions. Separate what she says directly, what an employee reports and what a witness assumes from a glimpse. Susan's final explanation depends on noticing that those are different kinds of information.
Sources (checked 19 October 2026): PBS: Moonflower Murders cast and characters, PBS: Moonflower Murders, episode 1, PBS: Moonflower Murders, episode 3, PBS: Moonflower Murders, episode 5, PBS: Moonflower Murders, episode 6
Read or listen to the book
Moonflower Murders (2020)
Anthony Horowitz’s novel behind the second television mystery.
Waterstones · print Audible · audiobookPaid links may earn commission, at no extra cost.
Rosalie Craig plays both Lisa Treherne and Melissa James, connecting the two story worlds.