Donna and Will's relationship ended when Will suffered a breakdown, but it was actually a cover for Will's work as a researcher at a secret government facility. It was there that (among other things) Will was working on an invisibility suit, with the help of Henry Wright. As for Donna, Will contacted her for research on cuttlefish, though Donna believed that he wanted to get back together with her. After Will got his research and used it to complete the suit, the calls to Donna stopped, and that left Donna enraged.
In an act of revenge, Donna took possession of the suit and used her invisibility to easily walk into Will's apartment after he opened the door. She took Will's pool cue and stabbed Will to death, and later in the episode, the evil Donna returned to the apartment in the suit and assaulted Castle and Beckett, while both sides were attempting to get Will's key card to the facility. The villainess escaped with what she believed was the card, but it was actually Castle's credit card that she took.
After Henry was taken in for the assault on Tom Talmadge, his statement that Will based the equations on cuttlefish research revealed Donna as Will's killer. Donna entered her office and saw Castle (who later found his credit card) and Beckett searching for evidence, while confronting her regarding her actions. After denying everything, Donna admitted to killing Will, accusing him of using her and lying to her. She later escaped just as Beckett was about to arrest her, and she put on the suit after hiding in a laboratory. The duo searched inside, and it was at that moment that Donna tossed beakers and other objects at them, while also turning on the gas and setting a fire in an attempt to kill them. After Castle put out the fire with a fire extinguisher, he and Beckett each began using one to find the invisible villainess. Beckett successfully subdued Donna and removed the suit, and following this, Donna was arrested.
- Camille Chen later played villainess Sheila Vestal on Lucifer.
- "What was I supposed to do? Let him get away with it?! He stopped calling, stopped texting! He used me and then pretended that I didn't exist! He wanted me to just disappear from his life, so I did...and he never saw me coming." (Donna Brooks confessing to killing Will Fairwick)