After a prep school guidance counselor dies monitoring a dance, Det. Mac Taylor (Gary Sinise) has a messy crime scene and too many suspects. Considering death took place in a chemistry lab, a student might be responsible. Mac and Det. Stella Bonasera (Melina Kanakaredes) are long time investigators. They know the first thing to determine is who found the corpse. When they simultaneously ask, Det. Don Flack (Eddie Cahill) points out they might be partnering too often.
Meanwhile. Det. Danny Messer (Carmine Giovinazzo) and Dr. Sheldon Hawkes (Hill Harper) process the body. Due to chemical spillage, Robert Greggs (Paul Keeley) is hardly pristine. Dr. Sid Hammerback (Robert Joy) later says Greggs died from ingesting acid.
Adam Ross (AJ Buckley), the quirky but brilliant technician, puts pictures from the students' cell phones together on the same screen. This just might be the evidence needed to crack the case.
Cracking the case is something Mac needs now. A serial killer posing as a cabdriver ("Like Water for Murder") is loose, but too many people with ties to the school want him to solve their case as a higher priority. Having Deputy Inspector Gerrard's (Carmen Argenziano The Young and the Restless) daughter attending the school cranks up the tension.
When Mac and Det. Lindsay Monroe (Anna Belknap) talk to Natalie (Kelen Coleman), they feel certain she knows more than she's telling. Gerrard stops the interview before they can learn much.
Lindsay and Mac talk to Natalie one more time. This time, Gerrard is in the room. When Natalie speaks to Lindsay alone, Mac and Gerrard soon hear a painful admission. The boy she had been dating a few times got her drunk, then slipped a date rape drug in her drink. The admission is agonizing for both father and daughter.
More shocks follow. He isn't a teenager, but 32! This was an elaborate setup to find teenage girls for sexual molestation. A sexual predator, Frank (Ayre Gross The Riches), masterminded the plan, but neither man has any remorse. The guidance counselor was killed after Natalie told him what happened to her. She wanted to ensure nobody else got hurt.