Nelly, an eight-year-old girl, has just lost her beloved grandmother and is helping her parents clean out her mother's childhood home. One day, her mum abruptly leaves, and Nelly meets a girl her age as she's building a tree house in the woods.

Saïd heads BRI, an elite task force of young police officers dealing with organised crime. As the new chief, Saïd tries to find his place in the team while trying to stop an all-out gang war from breaking out in Paris.