Ruth Negga stars as the titular character who escapes with her teenage son into the island where she was named after to hide from a violent crime, where she reunites with her estranged adoptive family.

Upon arriving at the island, the two burn her partner's body inside their car, but Bull struggles with guilt for killing his father.

After Daniel's family discovered their affair and Iona's pregnancy, she abandoned them and raised him in Glasgow, hiding from Bull who his true father was.

The island's traditional Celtic Christian community welcomes back Iona, however she awkwardly reunites with Elisabeth, who has married her childhood sweetheart Matthew and together they have a 14 year-old daughter Sarah, who is a paraplegic.

Sarah suggestively points Bull to her family's barn where they have sex atop a haystack, however he unceremoniously departs and leaves her there.

Iona returns to Daniel's house where she finally talks honestly with Elisabeth, who confronts her about her sexual relationship with her father and for hiding Bull's true paternity from him.