Why is MI5 hampering DI Maggie Hardcastle’s investigation into the assassination of ex-government lawyers?

When two ex-government lawyers are murdered on the same day, newly promoted DI Maggie Hardcastle finds herself deliberately misled by the security forces as she investigates. Further killings divert her from the truth, and Maggie comes to understand they are falsifying evidence in order to frame an innocent man. But uncovering the real criminals will result in many deaths, and she must make a harrowing decision: justice or silence.