Oh hi there, Motive. You're really doing awesome in the guest star department this season. First you bring us Laura Mennell, then here's Enuka Okuma. Wait, what do you mean this isn't a Rookie Blue crossover? Wait, what's happening to...okay, now you've gotten our attention.
Best-selling mystery author Lori Oliver - played by Enuka - is doing a book reading for a sizeable audience, who are hanging on her every word. In mid-sentence, she stumbles and thinks she's coming down with something. She's right; she's about to be dead. During the subsequent book signing, after meeting a guy who chillingly tells her "We've met before," Lori collapses but not before blurting out the phrase, "Gary, I'm sorry." Mystery Dude is identified as her killer.
While agonizing over her subpoena received last week, Angie brings Oscar to the crime scene, where Betty tells them that she thinks Lori was poisoned. Angie's response is to make a Clue joke. Smooth, Detective Flynn. As she and Oscar question Lori's manager Craig and her husband who happens to be named Gary, Mystery Dude - whose name is Dustin Hess - goes home to his collection of Lori Oliver books and asks himself, "What have I done?" We find out that before he was poisoning people, he was a frustrated writer teaching a college class that apparently takes place in a giant concrete box and telling his students, "Lori Oliver writes books for people who don't read."
Lori's assistant Parker shows the detectives boxes of hate mail, and reveals that Lori was having an affair with Craig. Betty freaks out Lucas before telling him that what killed Lori was mercury - as in, the stuff that's in thermometers. As it turns out, dimethyl mercury is a pretty nasty poison. And when our heroes begin to question Gary about the affair, they realize that he's been poisoned, too. He's a dead man walking, so he spills details about his relationship with Lori and blames Gary. Dustin, meanwhile, goes back to the scene of the crime.
Team meeting time! The squad tries to figure out how Lori and Craig were poisoned, enlisting the help of Parker, who suggests it might have been put into her perfume. "I just bought her new bottle," she says, then adds in an increasing panic, "I tried it when I bought it for her." Uh-oh. Craig also talks about Lori always losing her pen, which is found in his car. Bonus points to the dude who will not be able to use them.
Angie crashes another conversation between Cross and Samantha to explain what's going on. Samantha is not thrilled about this, but her boyfriend doesn't care. Before Angie can run away, Samantha tells her that the defense in the League of Nations trial just subpoenaed another witness named Julie - the daughter of Alfonso, the guy that Angie shot ten years ago, thus creating this whole cluster between her and Cross. She leaves, and Angie and Cross stew on this news for a moment before having to go stop Gary from attacking Craig.
In the past, Dustin's ego is further wounded when the college brings in Lori to talk to his students, who give her a welcoming round of applause. Present day, he's being haunted by her ghost, who bitch-slaps him for stealing her pen, and his student Doug will not shut up about the murder. While Dustin cooks up more poison in the chemistry lab to use on Doug, Lucas talks about what he found in Lori's hate mail and that points the team to - you guessed it - the college. Who does this guy think he is, Walter White?