Costello is engaged by euthanasia campaigner and solicitor Aisha Wiseman to defend Sarah Stephens, accused of helping her tetraplegic daughter to die. Reader is prosecuting and brings in Sarah's ex-husband and their teensge son as witnesses. Whilst two expert witnesses appear to endorse this and Sarah's defensive attitude towards her family make her less than cooperative, Costello realises that she may be covering for somebody else.

Costello takes on the biggest case of her life, defending Mark Draper, a repeat offender on a murder charge at the Old Bailey. However, when the case starts to fall apart, she must remain focused on the trial as both personal and private pressures escalate. Meanwhile, it's decision time for the pupils, and the fate of chambers hangs in the balance.

Reluctantly representing Farr, Costello vows to do her job, but she will not allow him or Micky Joy to get under her skin. Fighting shifting power plays and an untenable defence, she begins to feel that Farr is not getting a fair trial and suspects foul play. Setting a dangerous trap, can Costello uncover the unbelievable truth? With the date of his tribunal looming, Reader focuses all his efforts on prosecuting Farr, despite the apparent lack of support from CW. In the face of an uncertain future, Lamb finds himself trapped by Micky Joy and loyalties are stretched to breaking point.

Costello is now a QC, but she learns fast that it can be lonely at the top. At Shoe Lane Chambers, times are tough, and senior clerk Billy Lamb struggles to keep things afloat while Reader tries to be stoic about his unsuccessful Silk application. Costello is defending Brendan Kay, a member of one of London's biggest crime outfits, accused of blinding a man who allegedly disrespected their 'number two'. Costello's job is further complicated as Kay's solicitor, Micky Joy, is an old school player of the system who walks both sides of the line. Also up against notorious QC Caroline Warwick, nicknamed Lady MacBeth for her uncompromising approach, Costello feels herself pulled by the various political forces around her, and realises to win she will need an all or nothing defence. Meanwhile, Reader undertakes a sensitive case with new solicitor George Duggan.

Costello goes to lockup to visit old flame Sean McBride, who is charged with fatally shooting gangster Jimmy Monk. Despite an obvious bias, she represents him with Warwick prosecuting though she comes close to exposing her position and needs Reader to support her.

Costello represents Captain Ed Ryan in a court-martial. He's accused of disobeying direct orders, resulting in the death of a 19-year-old private. Under the watchful eyes of a distraught family who want answers, Costello navigates the army's legal system with the help of Captain Cassidy, Ryan's unit defence officer. Doggedly seeking the truth, Costello slowly peels back the layers of past and present wounds, revealing the shocking reality of modern combat. Meanwhile, temptation is never far away for Reader, who is warned by Lamb to keep his relationship with Duggan purely professional.

Costello defends a vulnerable teenager and the pupils are given their first opportunity to stand up in court, taking on a low-level magistrates' case. However, as the cases progress, both Costello and protégé Nick Slade find themselves in challenging positions. Can Costello defend her client without endangering not only her Silk application, but also her career?

Reader is in Oxford, prosecuting three students accused of sexually assaulting a waitress. Up against an intimidating defence counsel, he begins to waver, but a chance encounter with an old friend helps him discover a passion for prosecuting he didn't know he still had. When the case is in danger of collapsing, Reader is faced with one of the biggest decisions of his career. Meanwhile, Costello defends Ricky Armitage, who trashed his local mini-mart and attacked the shopkeep. His level of violence and lack of remorse is astounding, yet Costello tries to divert the self-destructive path that the young man was fated to walk.

Costello is approached by solicitor Dani Kakwani, whose nephew Rashid is wanted for extradition by US authorities for his supposed role in a bomb outrage at an Arizona university. The American embassy is keen to offer a deal in order to hurry the process but Costello believes their information is false and that he was not where they claim.

Having just won an important case, Martha Costello is given two new cases for the next day, with a chance of getting a good reference for her Silk application. Under huge time pressure, she has to prepare overnight. But will she be able to satisfy both the personal and the professional demands placed on her? Or will she be forced to sacrifice one case in order to win the other?