A newscast reports that it's the 7th day of heat wave, temperatures are over 100, and there's no relief in sight. In a parking lot, a woman walks to her car, looks for her keys and sees a baby in a nearby car, looks in and starts calling for help. A man breaks the window. Grissom and Catherine arrive and Brass tells them that there's no registration in car, but the parents can't be too far, it's a shopping plaza. He comments that it's 108 degrees outside, inside the car is 145 degrees. A man arrives at the car; Brass asks if he has a son.
In the parking lot, Catherine is disgusted by parents who leave their children in cars; David says that the body's fully rigid, and because of the heat he can't say when exactly the baby died. Grissom tells Catherine that with all the people around, she should treat it like the baby is still alive, so Catherine wraps it up and takes it to a coroner's van.
On the lake, a ranger talks to Nick and Sara; he pulled a female body from the lake, and had to move the body so that she wouldn't float away. They figure she's been dead less than 24 hours, but she didn't drown, she has a head wound.
Catherine watches while Paul Winston tells that he was supposed to drop Joshua off. He explains that his wife usually takes their son; he was busy and got a call on his cell phone, was thinking about work, and forgot about his son in the back seat because he's not the one who usually drops off the baby. He tells them that he's an agent for the gaming commission, and his wife works just as much as he does; she's currently in LA on business, and he's worried about what he's going to tell her when she gets back.
At the morgue, Sara photographs the body of the girl and finds something green in her hair. Nick notes that there are no identifying marks, but finds a locker key in her shorts. Nick comments that he's put in for a promotion, and asks Sara if she's heard anything about her application; she hasn't. David comes in to wash the body, and when he does he notices that the water beads on her skin, and there's a mark. Dr. Robbins tells them that the girl's cause of death was a blow to the head; the sexual assault kit was positive for semen, but Dr. Robbins says that it wasn't rape. He also tells them that she ate two to four hours before her death. The ranger calls and tells them that he's found the locker.
Catherine talks to the Chief DA; he wants to know everything about the case, about the child, the time of death, and what time Mr. Winston starts work. Catherine tells him that the baby was in the car for about 90 minutes. The DA tells her that the case will go to court, and he wants the father behind bars, so that in the future, parents will check before they leave the car; he wants everything documented.
They examine Mr. Winston's car; the baby seat is fastened directly to the car seat; when Catherine removes it, she notes that there are no indentations on the leather, and Grissom points out that this wasn't the primary vehicle for transporting the baby. They find a red liquid on the floor of the car, but it tests negative for blood.
In a house, Warrick takes pictures of the body of a man in a chair, Grissom comes in and tells Warrick that Catherine is doing baby case on her own, so he'll be helping Warrick. Warrick brings him up to speed on the case: the body is Wesley Jones, age 33, there are no signs of forced entry, and the Super found him in the chair like this. Grissom notes that robbery doesn't appear to have been the motive. They find a diet milkshake and a fitness magazine, as well as cookie crumbs. Grissom notes a work-out schedule on the wall, and diet pills on the counter, and comments that these things aren't a good combination with the heat. Warrick speculates that Mr. Jones might have died of heat stroke.
Greg tells Catherine that the red substance from the car is cough syrup.
At the lake, Nick checks the locker, and finds slot machine tokens and student identification card in the name of Sophia Renatta, aged 20; there's also an XXL t-shirt, which Nick believes belongs to a boyfriend. Nick and Sara talk to the wave runner rental clerk who remembers the girl and 'her punk boyfriend,' commenting that she was a pretty girl with a problem, and the 'punk boyfriend' took advantage of that; he adds that from what he could tell, the two of them had just met. They check the rental records and find that Mark Young rented the wave runner, but didn't use it until almost three hours after renting it.
Catherine and Brass talk to Mrs. Winston; she's very upset and tells them that she asked her husband to do one thing, and he couldn't even get that right. She tells them that she's a horticulturist, and had business in Anaheim, so she moved the baby seat from her car to her husband's before she left. Catherine asks if Joshua had a cold; Vicki Winston tells her that he had a slight cough, and she'd given him some cough syrup the night before, but told her husband to give him some more if he needed it. The officers lead Paul Winston by outside the room; Vicki sees him and runs out, begins screaming at him, hitting him and crying.
Sara and Nick talk to Mr. Young and ask where his son Mark is; he tells them that he last saw Mark yesterday morning when he ate breakfast and then left for his job. He tells them that Mark isn't a criminal. They ask for permission to search his home, which he grants. They remove some items from the house and ask if there's anything from which they can get a DNA sample; Mr. Young is upset that they're treating his son like a criminal, but they stress that he's missing, and was last seen with a girl who's now dead.
Dr. Robbins tells Warrick that Mr. Jones body showed no signs of drugs, he didn't have a heart attack, and he wasn't dehydrated, so it wasn't a heat stroke that killed him. He points out Mr. Jones' finger tip, which has a mark on it that Warrick had noticed; he says that it shows electrocution. He tells Warrick that it was low-voltage, but it affected his heart.
Hodges tells Sara that the substance on the girl's her skin was suntan lotion; Sara comments that it can't be Renatta's because they found suntan lotion in her locker and it was a lower SPF. He tells Sara that the green substance in her hair was algae, but the kind that would be attached to rocks at the bottom of a lake. They speculate that she fell from a cliff into the lake, hit her head, and Mark Young left her there. Sara and Nick check the lake charts for algae, but find that it's everywhere in the lakes; Greg comments that it could take months to find, then asks if the body washed up near Windmill Cove. They tell him that it did, and he informs them that Windmill Cove is near two other coves that are popular make-out spots.
Catherine checks the car again, and finds a bag in the back seat with clothes in it, as well as a baby blanket with the name Howard Aston Winston on it. Brass tells Catherine that Howard Winston would have been three years old; he died of an MS-like disease called Tay-Sachs. Catherine comments that the Winstons never mentioned having another child; Brass comments that losing two kids is tough.
Warrick goes over the chair in Mr. Jones' apartment again, but doesn't find anything. Archie checks the remote control for the TV, and comments that it's powerful, but didn't kill him. Warrick checks the phone and finds that it's burnt out, and comments that this is where the electricity came from; he guesses that there was heat lightning hit the phone just as Mr. Jones picked it up, and that's what electrocuted him.
On the lake, Nick and Sara go to the coves that Greg told them about and find the wave runner and a blanket; they test it for blood and find none, although there are signs of sexual activity. Nick wonders why Mark Young didn't take the wave runner back; the ranger tells them that there is an access road that could have been taken, but in this heat, that would be unlikely.
Warrick checks for lightning strikes, and finds that there have been none in the last three days, so his theory of lightning causing the electrocution is out. Nick comments that maybe the phone was the way that the electricity took out rather than in. Warrick and Archie check the scene again, assuming that the electrical current went out Mr. Jones' right finger, which means that it came in on the left. Archie comments that the point of entry had to have been somewhere else, as the left side is blocked by a fan. They note bare foot prints on the floor, and follow them to an octopus of wires plugged into a wall socket.
Nick and Sara follow footprints and come to a cliff which would be used for diving; they note that it's a 50-foot drop. Nick comments that it looks like fun when you're young. They assume that Renatta went over the cliff, possibly unwillingly, and hit her head in the water. Nick says that if this is true, then the evidence they're looking for is in the water.
Greg brings Catherine the results of the dosage of cough syrup that was in Joshua's body, which was no more than what was recommended. Catherine comments that with the heat, the baby would have fallen asleep, and Mr. Winston simply forgot about him in the back seat. Greg also tells her that there were pesticides in the baby's body; Catherine is concerned about this, and wonders how pesticides got into his body. She and Brass go to the Winston's home with a warrant; they search the gardening shed for supplies that would contain pesticides.
At the cove, Nick and Sara instruct the divers what to look for, and where.
Catherine finds some bottles in the gardening shed, one of which contains the chemical that they're looking for, and is marked as poison.
At the cove, one of the divers comes up and tells Nick and Sara that he's got a picture that they should look at; it's a picture of a dead male at the bottom of the lake; Mark Young.
Catherine gives the bottle of pesticide to Brass, who then takes Mr. and Mrs. Winston to the station for questioning. Mrs. Winston tells them that they said nothing about their other son because the whole experience was very hard on them. Mr. Winston tells them that he's guilty, he's negligent, and they should just leave Howard out of it. He adds that both he and his wife went through hell watching Howard deteriorate, and the police should just let him rest in peace. They decide that they need to get a warrant to exhume the body of Howard Winston to test it for pesticides.
Dr. Robbins tells them that Howard had Tay-Sachs, but other than that, the body seems to be in order. Catherine draws fluid from the body and has Greg test it for cough syrup and pesticides; the tests are negative, showing only an anti-inflammatory drug, which would be normal for a child with Tay-Sachs. Catherine wonders why a couple who didn't kill their first child who was dying would kill their second child who was healthy?
Catherine talks to the Winston's family doctor who says that he was with the family when Howard died; he tells Catherine that both parents carried the Tay-Sachs allele, so having a second baby was a real chance. He adds that Vicki Winston called him a couple of days before Joshua's death and brought him in to be checked for Tay-Sachs; even though she had tested negative during pregnancy, Joshua was showing signs of Tay-Sachs. He tells Catherine that he did the tests, but the results aren't back yet.
Dr. Robbins tells Nick and Sara that there's no bruising to Mark Young's body; the only thing wrong is that he has a broken ear-drum. Nick speculates that Sophia Renatta went off the cliff first, and when she didn't come back up, Mark jumped in after her, and dove down repeatedly looking for her. Nick decides that it was an accident, and Mark was trying to save Sophia. Sara comments that they can't prove anything, but the case is over.
Warrick tells Grissom that there was a water stain that ran across the floor of Mr. Jones' apartment, and a plastic bag in the kitchen garbage that had held an ice block. He speculates that Mr. Jones had made a 'swamp cooler' using the ice block and the fan. When the ice melted, the water from it travelled across the floor to the octopus of wires, and Mr. Jones electrocuted himself.
Nick tells Mr. Young that Mark died trying to save someone's life. Afterwards, Sara asks what he's doing, and is upset that Nick told Mr. Young that his son died a hero. She tells him that he shouldn't have said that when the evidence didn't support it.
Catherine talks to Mr. and Mrs. Winston again, asking if Joshua was ever with them in the garden. Mr. Winston says no, but Mrs. Winston says yes, he was; they'd had a fight about whose turn it was to watch the baby, and she had Joshua with her while she worked in the garden. When asked, Mrs. Winston can't remember if she washed her hands before picking him up. Catherine mentions the trip to the doctor's office, then tells them that repeated exposure to certain pesticides can mimic the symptoms of Tay-Sachs; she suspects that Mrs. Winston's use of them is how they got into Joshua's system. Catherine then speculates that after taking Joshua to the doctor for testing, they decided not to wait for what they felt were the inevitable results, so they gave Joshua cough syrup to put him to sleep, and then left him in the car until he died. Mrs. Winston tells them that they have no idea what it's like to watch your child die; Mr. Winston tells them that they couldn't go through that again. Catherine tells them that the test results have come back; Joshua tested negative for Tay-Sachs, which means that they killed a perfectly healthy baby.
Catherine goes to her car, gets in and checks the temperature: 135 degrees. She shuts the door, and watches as the temperature rises to 136... 137...