Arthur is one of the good guys. A CPS solicitor working to prosecute those who do wrong.
But when the system fails and victims do not get justice Arthur realises other methods are needed to punish the guilty.
He enters into a pact with a veteran detective to ensure the streets of London are safer. However, the pact requires trust and secrecy and when you cannot rely on others the path he has taken becomes fraught with danger to his life and liberty.