Nathaniel is an observant teenager growing up in post-World War II London. After his parents depart for a supposed assignment in Singapore, he struggles with feelings of isolation and abandonment. To cope, he seeks out structure and connection outside his home, working at the Criterion Restaurant and assisting with illicit smuggling operations on the Thames.

Rachel is Nathaniel's nearly sixteen-year-old sister who also suffers from the shock of their parents' sudden absence. She experiences epileptic seizures, which requires specialized care from the adults around her. Finding an escape in the arts, she develops a deep interest in theatre and begins working as a stage prompter in Covent Garden.

Rose is the mother of Nathaniel and Rachel who leaves them under the pretense of joining her husband in Singapore. During her time at home, she acts playfully with her children, calling them by pet names and participating in dramatic performances. Her true occupation involves secretive work that keeps her away from her family.

Walter, nicknamed "The Moth" by the children due to his quiet, flitting presence, is a humble but mysterious man. He works at the Criterion Restaurant managing immigrant staff and routinely invites an eclectic group of strangers into the Ruvigny Gardens home. Though Nathaniel suspects him of criminal activity, Walter works diligently to keep the siblings safe.

Norman Marshall is a charismatic former boxer who makes a living smuggling greyhounds on a mussel boat and fixing dog races. He operates with a brash, spontaneous energy, driving lorries and commanding the unlit canals of the Thames. He takes Nathaniel under his wing, employing him on the river and teaching him how to survive independently.

Agnes is a seventeen-year-old girl from a working-class background who becomes Nathaniel's first love. She adopts the name "Agnes Street" after an empty property they explore together. Constrained by her lack of money, she dreams of discovering a metaphorical large pearl that will elevate her circumstances.

Sam is a dependable farmer and neighbor who lives in the Saints, the area containing Rose's childhood home. He sells produce grown on former military airfields and possesses a deep, empathetic connection to the natural world. He provides a calm, steadying presence for Nathaniel.

Marsh is a naturalist and a childhood friend of Rose. From a young age, he takes an interest in building climbing and animal behavior. He hosts a BBC radio program and maintains an unpredictable, recurring presence in Rose's life, eventually introducing her to complex, dangerous work.

Olive is a worldly ethnographer and geographer. She possesses a highly attuned sensitivity to the environment and the emotional states of those around her. During evening walks, she teaches the children to listen to nocturnal animals, offering them a brief but impactful sense of maternal warmth.

Arthur is ostensibly a quiet scholar of French and agronomy who associates with the strange group managing the Williams household. He maintains a serious demeanor and actively monitors the safety of the children when unusual threats arise.

Mr. Florence is a beekeeper who associates with The Moth. He is known for using his bee smoker equipment in highly unconventional and illegal ways to assist the group's objectives.

Citronella is a fashion designer who forms part of the eclectic, shadowy network of professionals frequently hosted by The Moth during the postwar period.

Harry is an energetic dishwasher in his forties. He freely entertains the restaurant staff with colorful stories of his youth and romantic adventures, providing Nathaniel with an unvarnished education in adult matters.