Lynne Ramsay's latest film is one with gritty and horrific subject matter, with a tender heart centred around the growing bond between a broken, suffering adult and and innocent child, forced out of her innocence by her circumstances and those around her.

The film, an adaptation of Jonathan Ames' novel, follows Joe, an assassin suffering with PTSD from his military background and a painkiller addiction. Joe is quiet, seemingly numb to the ruthlessly violent acts he must take part in to track down missing girls for the highest bidder.

He lives with his ailing mother in his childhood house, and the relationship between them seems to be one of genuine affection, despite Joe keeping his job very much private. He cleans himself up, throws away his bloodied clothes and makes sure he is presentable to eat with her, though she must suspect he is not working a "steady job" based on his secrecy.

But then Joe is told by his middle man, Angel, that their boss McCleary has a job for him and he must visit him directly. Joe discovers Angel is "out" because his son accidentally spotted Joe leaving his apartment, jeopardising their operation.

McCleary gives Joe the task of rescuing a senate-hopeful's teenage daughter after she was kidnapped and is being abused in a brothel by wealthy businessmen.

Though this job sounds like any other at first, it soon spirals out of control and the once collected assassin must summon resources to save a young girl who has been brutally treated by all those around her.

Their bond is central to the drama - even though it is at least a third of the way through that the two meet - as the man whose own past causes him to function in a haze becomes the closest to a father that the young girl has ever experienced. She, in turn, allows him to focus his mind out of his own crises and onto the care and protection of someone more vulnerable than he.

Lynne Ramsay's direction is slick and beautifully lingers on the face of her protagonist, played by Joaquin Phoenix with a thoroughly nuanced sense of anguish. The easy job quickly turns to nightmare and Ramsay perfectly pitches the changing pace of his nightmare, showing every millisecond of the action in the cold light of the street lamps.

Jonny Greenwood's score is also incredibly well suited to the action, as its pumping bass and electronic sounds overtake the audience's ears and create a tense feeling of uneasiness as the drama unfolds.

The lead players are especially watchable, and the young Ekaterina Samsono, as the traumatised teenager Nina, is electrifying as she portrays a girl who seems younger than teenage as her innocence and confusion are played with clinical observance.

Though the subject matter is by no means easy, this feels like an important film in the career of its ever-brilliant director, who is placing herself among the best of British filmmakers.