Set in the morally grey world of assisted suicide, "Mary Kills People" is an intense, controversial, and thought-provoking drama.The series follows Dr. Mary Harris, a single mother and emergency doctor by day, who also moonlights as an underground angel of death — helping terminally ill patients who want to die and slip away on their own terms. So far Mary has managed to stay under the radar; but business is booming, and her double life is getting complicated. When her world starts to unravel, Mary realizes she’s going to have to fight dirty if she’s going to stay in the killing game.
Des is happy to reteam with Mary following his stint in prison - but when a routine case leads to a devastating outcome, Mary questions whether she was better off fighting the good fight without him in control and unchallenged.
When Olivia attempts to blackmail Mary, she is drawn deeper into a deadly game of cat and mouse that has a surprising connection to Ben.
Mary and Ben decide to join forces and hide Travis at Ben's secluded trailer. But when Travis reveals a crucial detail about Ben's investigation, Mary is forced to reconsider their alliance.
Following the disappearance of Travis, Des worries that he and Mary may have crossed a moral line from which they can never return. After jeopardizing Ben's investigation of Olivia, Mary tries to make amends, which has repercussions for a patient in dire need.
All seems well for Mary, Ben has forgiven her, and she and Des have decided to move forward with the death retreat. The only nagging concern for Mary: her daughter Jess hates her and doesn't believe a word out of her mouth.
Mary secretly takes in patients, but one death doesn't go as planned.
Mary promises Des that she'll never put the hospice at risk again. Mary and Frances meet an elderly cancer patient. Jess realizes that something's wrong with Naomi.
Mary makes a horrifying discovery, sending Annie on a dangerous collision course with Frances. Meanwhile, Ben attempts to reconcile with Mary, and Des becomes attached to a difficult patient.
Reeling with guilt over Leah’s death, Mary turns to Ben for help and agrees to stop killing people at the hospice. But after watching one of her patients die an excruciating natural death, Mary recommits to her cause and hatches a plan to deal with Frances.
Mary and Ben take a calculated risk in trying to take Frances down.
When Dr. Mary Harris becomes romantically involved with an attractive patient, it sets off a chain of events that could destroy her life, her family, and her career – not to mention her illegal side gig, helping terminal clients end their lives.
After being intimate with Joel, Mary has second thoughts about ending his life; a client's son pulls a gun on Des; Kevin's relationship with a new woman gets intense.
Mary confronts Ben; Grady blackmails Mary into helping him break a patient out of the hospital; Ben and Frank get a new lead.
Ben follows Mary to her childhood home where he learns about her past; Grady approaches Mary with an offer; Des becomes a victim of police misconduct.
Mary seriously considers giving up her operations for good.