Four bodies are found in the Thames after a boat full of illegal immigrants founders. Realising they are dealing with a people-trafficking operation gone disastrously wrong, Leo, Harry and Nikki set out to discover if there are any survivors - or any other deaths - while also delving into the victims' past.
The team identify the four bodies found in the Thames, and discover two of them were a couple fleeing China because they wished to have a second child. Nikki realises their six-year-old daughter was also on the boat - and may still be alive somewhere. Meanwhile, the detectives close in on the gang running the people-trafficking operation.
The team take on three cases at once in between major hug investigations. Harry deals with a hit-and-run on a council estate, Leo investigates the cause of a fire that left a woman dead, and Nikki is called out when a bride-to-be collapses and dies on her hen night.
Harry, still devastated by the tragedy he witnessed, investigates the apparent suicide of a high-profile footballer - and discovers a link to the hit-and-run case. Leo is called in when a tramp is found dead on a night bus, and Nikki looks into an office prank that proved fatal.
Harry's burgeoning relationship with Nikki is jeopardised when his ex-girlfriend Penny is killed in a car crash. It looks like suicide at first glance, but DCI Ambrose uncovers evidence that the victim may have had an affair - as well as a secret child. Meanwhile, Leo examines the staged death of a conceptual artist.
Harry is determined to find the hugger, but he and Nikki are pushed farther apart when he forces her to re-examine Penny's body. Leo is puzzled by more contrary clues in Jimmy Triangle's bizarre death - and becomes convinced that someone is playing a twisted game.
When Leo is called to investigate the apparent suicide of 14-year-old Alison Garland, it evokes painful memories of his daughter's death and with the help of a child psychologist, he realises the girl was cuddled. The case is also linked to another incident in which a car salesman was killed with the same knife.
A third hug is discovered on school grounds and Leo realises the killings are connected to a sinister teenage pact. Meanwhile, Nikki proves her elderly victim died of hypothermia - but there is still a guilty party to be caught.
A teenage girl is found tortured to death at a London dogs' home, and it soon emerges another young woman is being held by the culprits, who turn out to be a unhappy team of animal activists seeking to destroy an experimentation facility. However, the case becomes much more complicated when Nikki is kidnapped at gunpoint by the group leader.
With Nikki kidnapped and another young woman being held hostage, Harry faces a race against time to find their captors, who are based deep inside the woods. Meanwhile, Lionel's hearing continues and Leo is forced to decide whether to follow his conscience at the expense of his best friend's livelihood.