Renowned heart surgeon Dr. Rachel Waters arrives at Community General, there to perform heart surgery on US Senator Jackson Cabot and use said surgery to teach it to other medical students. Mark himself is especially happy, as Rachel is a flame of his from his medical school days, and perhaps hopes to spark the relationship once more.
Unfortunately for him, Rachel has plans for her surgery on Cabot, and it ain't to help him. In the night before, she steals blood from a patient suffering from a staph infection and taints a catheter with it during the surgery. In enough time, Cabot dies soon after. Determined to clear the name of a nurse blamed for the error, Mark sets to investigate his former love.
"A Twist Of The Knife" displays symptoms of the following tropes:
- Asshole Victim: Senator Cabot killed two people and effectively killed a third in a hit and run and covered it up to save his own ass.
- Hero of Another Story: While Mark is snooping around Chapin's office, he ends up talking to a man there to see Chapin. The man is there to expose corruption he has discovered in the Department of Water and Power, including the embezzlement of over $3,000,000 over the past four years, involvement from the city controller and members of his staff, and even bribery going as high as the state's Lieutenant Governor.
- Nothing Up My Sleeve: Rachel is able to infect Cabot by smearing a piece of bandage with the infected blood on it and hiding it in her sleeve before her surgical gloves come on.
- O.O.C. Is Serious Business: In reviewing footage of a surgery like Cabot's that Dr. Walters performed in another hospital, Mark realizes that while she just threw away her gloves in the non-murderous surgery, she rolled them into a little ball in the lethal one, in order to conceal the infected blood cloth. This would ultimately undo Walters, as the gloves with the cloth would also contain her fingerprints.
- Reverse Whodunnit: This is the first time in the series where we know who the murderer is off the bat, and the trick is Mark finding out how to prove it.
- Spanner in the Works: The maid at Rachel's hotel who knocks over her original sample of infected material, thus forcing her to sneak into Community General for the staph blood she needs.
- You Killed My Father: Waters does what she did to get revenge for her daughter being put in a likely permanent coma as a result of Cabot.
