Bol (Sope Dirisu) and Rial (Wunmi Mosaku) are refugees fleeing with their daughter Nyagak from war-torn South Sudan. To cross the Mediterranean, they brave stormy waters on an overcrowded motorboat, along with fellow refugees traversing the dangerous open sea in search of a better life in Europe. Although they survive the treacherous crossing, their daughter and many others do not. When they are finally granted probational asylum in Britain three months later, the government assigns them a shabby house with peeling walls and dismal furnishings on the outskirts of London. They are given strict restrictions or they may face deportation. They experience racism and hatred from their tenement neighbors. They are met by their case worker Mark (Matt Smith), who tells them he hopes they are two of "the good ones".Bol tries to assimilate: he sings football songs, asks Rial to use utensils rather than her hands when they eat, and even changes how he dresses. Bol wants to prove to the government that he and Rial belong in the UK. Rial, however, clings to their culture. She retains their daughter's necklace, dresses in colorful clothes, and rather than using a table, sits on the floor to eat. Both Bol and Rial soon experience strange and disturbing phenomena in their new home, and see visions of Nyagak and a mysterious man, who both escape into the walls.Rial doesn't take long to work out what the evil in their house is: an apeth or "night witch". She tells Bol the story of a poor man in her village who accidentally stole from an apeth by the river. When the thief built his home, the apeth moved in with him and haunted him. Rial believes that an apeth has followed them and if they repay their debt, the apeth will bring Nyagak back to them. However it is not immediately clear what the "debt" is that they need to repay. Bol burns everything they brought with them, but the apeth continues to torment him and things deteriorate between the couple. He goes to Mark and requests new accommodation under the guise that their unit is infested with rats, but is unable to convince him. Bol tears apart the house looking for the apeth, which threatens the couple's chances of staying in the UK when Mark discovers the damage. However, this is fine with Rial, who tells Bol that she wants to leave. Bol locks Rial in the house before he summons the apeth himself, who calls him a thief and claiming that Bol has stolen a life that was not his to take. The apeth offers Bol a deal: his life for Nyagak's, but he outright refuses this offer, resulting in Bol being forced into a state of momentary catatonia out of anger.Rial manages to escape the house but finds herself inexplicably back in South Sudan in a familiar class room. She is reunited with old friends, who are revealed in a further flashback, to be victims of a horrendous massacre. Rial was able to survive by hiding. Bol finds her and the couple make their escape as violence grips the region, but find that a bus they need to board would only let people with children on. Desperate, Bol sees Nyagak in the crowd and abducts her, falsely claiming that she is their daughter. The couple get on the bus and escape, leaving Nyagak's real mother running behind the bus, as gunfire erupts. Later, many Africans are crossing a rough sea, when Nyagak and others fall overboard. Neither Bol nor Rial could reach her in time. Having accepted what they did, Bol decides to repay the debt to the apeth and tells Rial. Bol starts to let the apeth into his skin and Nyagak enters the room and returns to Rial. Rial chooses to save Bol instead of accepting this alternate reality, by slitting the apeth's throat.Later, Mark comes to inspect the house to find it repaired. Bol and Rial tell him they have chosen to stay and make it their new home. They say Rial killed the witch that haunted them, which Mark finds funny. Bol says they decided to live and go on with the ghosts of their past from South Sudan, including Nyagak.We see a vision of the couple in the doorway with other unknown immigrants, who look into another doorway full of people left behind. Then we see the couple standing in the doorway of their new home with a peaceful look in their eyes.