Helen is an 83-year-old widow who returns to her childhood town in England after living in Australia for sixty years. Deeply grieving the losses of her husband and son, she strictly isolates herself from her neighbors and denies herself basic comforts out of a sense of survivor's guilt. Her quiet routine of watching television and retreating into memories changes when she inadvertently brings a live mouse into her home.

Sipsworth is a small, quiet mouse who ends up inside Helen's home after being left outside in the trash. He possesses delicate features, including a soft coat and large ears, and displays a surprisingly calm demeanor around humans. He earns his distinctive name by delicately drinking water from a bottle cap.

Cecil is the friendly shopkeeper of the local hardware store. He maintains a helpful and conversational demeanor with his customers, readily offering advice on home supplies. He holds a deep respect for Helen's family due to a past connection with her father.

Dr. Jamal is a young, dedicated surgeon who serves as the Head of Cardiology at the local hospital. He has a background as a combat medic and holds great respect for his medical predecessors. He approaches unusual medical requests with an open mind.

Len is Helen's late husband, whom she met in Australia in 1960. He was an abandoned child who took Helen's last name when they married. He was a dedicated partner who occasionally accompanied Helen to the opera simply to make her happy.

David is Helen and Len's late son, who worked as a schoolteacher before passing away in a car accident. He was a gentle person who, as a boy, wanted a pet mouse rather than fish.

Helen's late father is a significant figure in her past. He survived a torpedo attack during World War II and was rescued by Spanish fishermen. He later worked as a teacher in Helen's hometown.

Dominic is the young son of the local librarian. He spends time at the library and has a keen interest in anatomical models and animals. He is eager to assist patrons and quickly takes a liking to Helen's unusual pet situation.

The local librarian is a welcoming and community-minded woman who helps manage the town's book collection. She encourages social interaction by inviting new patrons to coffee hours and facilitates research for curious residents.

Kathy is a nurse at the local hospital. She acts as the initial point of contact in the cardiopulmonary department and manages patient and visitor requests, connecting them with senior medical staff when necessary.

Dr. Preston is the head veterinarian at an exotic pet clinic in Oxford. She coordinates with human medical professionals to provide specialized medication for small animals.

The neighbor is a resident of Westminster Crescent who unknowingly initiates the central events of the story by discarding his daughter's old gerbil tank. He is observant of the activity on his street.