A group of high school students are partying at a friend's apt while her father is away. Amongst the group is Caitlin (whose apt it is), her ex boyfriend Billy, and his current girlfriend Alyson. At the party Billy offers Alyson drugs, a white powder with a "cat" logo on it. Friends Justin and Mekhi jump in with Billy to snort some and Alyson nervously does as well. Jamie and Renzulli on duty, are called to the scene of the party per a neighbor's complain only to discover Billy, Justin, Mekhi and Alyson unconscious. The boys are dead but thanks to Jamie's CPR, Alyson survives. Meanwhile the Reagan’s debate legalizing drugs and all the pros and cons during family dinner. Danny and Jackie are on the case, and go to see Principal Jesuit Father Leo, and Vice-Principal Carol Hoffman at St. Angelus Catholic High School where the kids attended in an attempt to find the source of the lethal drugs. After identifying the drug dealer they find the drugs but realize the drugs were pure and someone has poisoned them and therefore Danny and Jackie figure out it is a murderer they are looking for, not a drug dealer. Later, having recovered, Alyson is out jogging and a driver attempts to run her down. Suspecting Caitlin of attempted murder, they search her lab space in Chemistry class and discover potassium chloride, which when mixed with heroin can stop the heart immediately. She openly hated Billy and Alyson, but she has a strong alibi and they do not have enough evidence to detain her for murder. She also reveals that Billy was cheating on her with a mystery woman. Alyson's description of the car that tried to run her down includes a school bumper sticker. Upon looking at all the cars at the school, they find one that meets her description with a bumper sticker freshly removed. It is Vice Principal Carol Hoffman. As it turns out, she was having an affair with Billy and could not stand that he was with Alyson and decided to kill him by poisoning his drugs. She tainted the drugs, not expecting him to share the drugs with his two friends. They arrest her for all three murders and attempted murder of Alyson. Meanwhile, Erin and Frank attend a charity gala that she has officially taken the place of her mother as the head, a position that was passed down by her grandmother to her mother and now her; an emotional transition displaying a side of Erin that Nicky had not seen in her mother before. We end on traditional Sunday dinner at the Reagan's, except this time Nicky orchestrates the entire meal consisting of her mother's favorite foods, and in honor of her grandmother.