The Watsons are no strangers to sibling placements, but when Casey takes the call from her supervising social worker one frosty January morning, she can instantly tell from the tone of her colleague’s voice that there’s a complicated case ahead. And she’s right. A four-day-old baby boy called Tommy - born in prison - plus his four-year-old half-brother, the lively Seth. A month later, the very moment she gets out of prison, the boys’ mother - a 19-year-old called Jenna - also follows.

A 5-year-old boy, Ethan, is brought to Casey in the middle of the night after the sudden death of his young mother after a drug overdose. Estranged from her parents, Ethan’s mum had been abused by her ex-partner, and began taking drugs to cope. Ethan is obviously lost and bewildered, and regularly wakes up screaming for his mum in the night. He begins to lash out at other kids at school and his behaviour becomes more volatile.

Casey's Unit is, as ever, full of troubled, disaffected pupils, and new arrival Leo is something of a conundrum. Thirteen-year-old Leo isn't a bad lad - in fact he's generally polite and helpful, but he's in danger of permanent exclusion for repeatedly absconding and unauthorised absences. Despite letters being sent home regularly, his mother never turns up for any appointments, and when the school calls home she always seems to have an excuse.

Casey Watson is back, doing the job she does best - rolling up her sleeves and fostering the children who, on first meeting, seem like hopeless cases. But when she meets Miller and discovers the truth about his disturbing childhood, even Casey begins to doubt if this child will ever be able to accept love. Found naked and alone on a railway track, Miller was just five when he was first admitted into the care system.

Harley, 13, has been sectioned under the mental health act after attempting suicide. She was spotted climbing the railings on a footbridge that crossed a busy motorway and pulled to safety by a member of the public. After six weeks in hospital, social services are looking for a short-term placement so she can be kept safe while family therapy takes place. Harley has a family - a widowed mother and an older sister, Milly, who left home with her long-term boyfriend just over a year ago.