Maxine Peake is Martha Costello, in her thirties, single, passionate and a defence barrister applying for silk. Innocent until proven guilty are four words she lives by. But how does this fundamental principle stand up to examination by clients who are sometimes good, sometimes bad and sometimes evil? Martha is faced with challenging cases and surprising clients. Her beliefs and prejudices, her conscience and her faith in the criminal justice system are tested to the limit over the course of the series. Joining Martha is Clive Reader, played by Rupert Penry-Jones. He's funny, gifted and dangerous. The same age as Martha, they're called to the bar together. Both are applying for silk – how they perform in court is vital to this process and Clive knows how to play the game. Neil Stuke plays Billy, the senior clerk at Martha's chambers. He is dedicated to Martha, even if secretly he believes that she hasn't got a chance of being made a QC. Nick Slade, played by Tom Hughes is Martha's pupil. Niamh Cranitch (Natalie Dormer) is assigned to Clive. Both are thrown in at the deep end and, as "baby barristers", must sink or swim. Only one pupil will be taken on as a member of chambers at the end of the series. Who will it be and why? And how will Martha and Clive influence the outcome?