Detectives Flack and Angell bring in a young man named Todd Fleming for questioning, but Angell leaves Flack to handle the interrogation while she helps deal with an unruly suspect in the bullpen. No sooner is the suspect subdued than Flack is calling for help--Todd is on the floor, seizing. Flack administers CPR and tries to revive him, but the young man dies. Despite Mac's concern, Flack opts to talk to IAB officer John Malley, until it becomes clear that the lieutenant thinks he used excessive force. Flack calls for a union lawyer, The story flashes back two days, to the beginning of the case, when a skateboarder stumbled cross a severed foot in a pile of trash. An arm is found in a dumpster blocks away. Both limbs are found near bloody Christmas wrapping paper. Sid determines the victim died the previous night, and Lindsay brings in a severed hand to add to the collection of limbs. Sid is able to get a fingerprint match from the hand to a man named Vince Nelson, a wrestling coach at Hillridge High. Stella and Lindsay visit Vince's widow, Amalia, and she tells them her husband had no enemies. She last saw him the previous morning, but had gone to bed before he got home from his business class. She does recall one strange incident the day before: she noticed two people lurking on the roof the day before. Stella and Lindsay go up to the roof and find a small pool of blood, which Stella takes back to the lab and identifies as alligator blood. Flack tells Mac that Vince wasn't registered in any business classes; he lied to his wife. He also withdrew $200 from an ATM the night he died.
Sid works on the newly-recovered torso and shows Danny and Lindsay a small gold disc he found on the body which Lindsay identifies as an acupressure magnet, used to help people quit smoking. Sid notes that Vince's lungs were in perfect condition. Sid also shows the CSIs he found sawdust on Vince's torso, and Danny posits that the murder weapon may have been a chainsaw. A print off the acupressure magnet leads Flack to Tanda Love, a salsa instructor with a record for solicitation. She tells Flack that Vince was taking salsa lessons to surprise his wife on their anniversary, and Flack is able to corroborate her alibi. Hawkes brings Mac a disturbing find: pictures of under-aged boys in an e-mail Vince sent out to seven of the students on his wrestling team. Mac is disgusted by the pictures, while Hawkes wonders why the man would make such an incriminating move. While Flack goes off to question the seven students who got the e-mail, suspicion lands on one, Todd Fleming, when Stella learns his science project for the school fair involved alligator blood. Flack and Angell speak to the nervous young man, but when he evades their questions, Flack says they're going downtown to the station...which leads to the fateful interrogation. As Todd mutters that, "this wasn't supposed to happen," he gets more and more agitated until starting to seize up. After the boy's death, Sid determines hypoxia caused his heart to stop, but that there were no illegal drugs in his system. Mac is certain Flack didn't kill the boy. IAB puts Flack on modified assignment, and Angell is upset when she finds out during her own IAB questioning that the department is aware of her romantic relationship with Flack, which she warns him will discount her testimony.
An alibi rules Todd out as a suspect, but when Vince's head is recovered, Sid determines he was killed by a blow to the head and also offers a new lead: a fleck of dried blood in the wound. The blood is matched to a rapist named Johnny Holt, who denies even knowing Vince. Lindsay is able to back him up when she finds the dried blood is three months old, meaning it must have come from the murder weapon. Mrs. Nelson continues to insist her husband was a normal man, and wasn't into child pornography. Hawkes and Mac discover Vince's firewall was breached and realize someone else must have sent the e-mail from Vince's account. Recalling Amalia's claims about two people on the roof, the two go to check it out and discover sawdust beneath the melting snow. Danny finally identifies the sawdust as coming from a beech willow tree, which is native to Flushing Queens, where one of the seven students lives: Kyle Sheridan. Kyle admits to, along with Todd, sending the e-mail after Vince changed his weight division and cost him a wrestling scholarship, but denies killing the coach. When he mentions his father, Alex, is a court officer, Mac realizes he's their killer. Alex used his baton three months ago to subdue Johnny Holt in court, and a visit to his house reveals sawdust, wrapping paper identical to that found with the body parts and blood on the basement wall. Mac and Angell arrest Alex who tells them he was disgusted after he saw the e-mails the coach sent his son. Alex beat him to death and, in an attempt to beat a murder charge, cut him up with a chainsaw and spread the body parts around the city. The case closed, Lindsay brings white roses to a grieving Amalia Nelson, while Mac offers Flack some good news: he's been cleared of the charges. Todd died from an overdose of an antidepressant.