Back at the lab, Calleigh and Horatio are reconstructing Stephanie's death, putting up a wooden frame and plastic sheeting to catch the detritus and packing a rifle with plenty of powder to simulate its kick upon firing. Calleigh positions a dummy, saying, "Usually suicides lean forward to get a better shot." Horatio muses, "That would create spatter directly above, but in this case, the blood is back here, so she needs to be back." Calleigh positions the dummy so that she's reclining on the couch. Mmmm, napping on the couch. I could get behind that. Horatio then leans down between the dummy's knees, positions the rifle, and fires it. Sure enough, it spatters to the back. After a loving close-up of the plastic carnage -- which really isn't an improvement over Stephanie's face -- and the confirmation that Calleigh used boiled noodles to simulate brains, the two conclude, "She's either the most relaxed suicide on record, or she was taking a nap." Well, there goes your Mom-goes-on-a-killing-spree theory, because how's she going to pull the trigger while napping? Someone else shot her.
Minutes later, Delko is pulling out Speedle's stenciled carnage diagram and saying, "The blood on the blanket in the doghouse proves the toddler was there after someone was injured." Horatio says, "Right, but her feet were drenched with the blood of all five victims, though." Speedle corrects him: "Not until after she was hiding in the backyard. The transfer from the blanket came from one bleeder -- her mom." And the Mom-on-a-spree theory has one more nail driven into it. I hope Horatio's prepared to grovel when he breaks this news to Stephanie's sister; I hope he breaks it to her, period. I also hope that maybe some of the other CSIs stop and think about how they have broken Olympic records in the Long Jump To Conclusion. Clearly, I'm hoping in vain. Anyway, Delko and Speedle conclude that there's no chance Erin got to the doghouse under her own power; someone carried her out there. Horatio orders the boys to find out who had glyphosphate and soil on their shoes. Both Luke and Jason do, so the next step is to see which one of them is capable of carrying the toddler out, and which one is likely to have snapped and begun murdering his family members.
Speedle's doing his part by checking the clothes for bloodstains while Calleigh watches; I get the feeling from their interaction that she's looking to pump him for as much lab savvy as he's got, as she's usually asking him questions and not vice versa. She asks why Speedle's focusing on pants and not shirts, and Speedle replies that "any spatter from a vic is going to be lost among the primary." Calleigh counters that it's hard to get spatter from a long-barreled shotgun. Speedle sits bolt upright and says, "Unless we're not looking for spatter." Calleigh checks the lens and notices several neat round dots of blood. She says, "That's transfer." Speedle recognizes the pattern: "That's tread, from the toddler's footies. The question is, whose blood is it?" Cue Horatio coming downstairs to get results from Speedle; Speedle tells him the transfer stains on Luke's pants come from Stephanie. We go into Horatio's voice-over as we flash back to the scene: "Instinctively, he picked up the toddler, thinking the killer might still be in the house. He did it after Mom was mortally wounded." That Luke had some serious presence of mind; it's not every kid who'd walk in on his mom with half her face blown off and immediately think of protecting his kid sister. Horatio wheels off, presumably to launch a group meeting. Speedle tacks up Exhibit A: The Dirty Dishes, and says, "Mom and Dad aren't doing their jobs, so the son steps up to the plate." Horatio adds, "Luke tries to bring his siblings lunch and walks into a massacre." Delko asks, "Once Luke's in the room, why not save the infant? The swing and the bassinet were in the room." Calleigh says, "Judgment call -- who can imagine shooting something so small?" Megan adds -- NOTHING! She's not here! It's as if the minute the news about Kim Delaney's departure broke, a team of editors was called in to begin excising Megan from episodes. It's pleasant -- no more typing [beat] all the time -- but somewhat unsettling. I imagine old Soviet ministers looking at group pictures from which coworkers had been erased might know the feeling. Horatio speaks next: "While Luke is saving the toddler, the killer goes after his next threat: Timmy." Delko comments, "Nine-year-old boy -- some threat." Speedle says, "Luke comes back in, tries to save the infant." Horatio continues, "But he dies trying to get the infant out the front door, and why does that happen?" Delko says, "Dad's blocking his path." Horatio counters, "Dad's been shot in the back, and his wound is not self-inflicted." Calleigh and Speedle suggest that Jason and Luke struggled over Max, and in the contretemps, Luke managed to shoot his father in the back. This is more or less confirmed when they look at Max's little romper and notice three small gravitational droplets of Jason's blood, which means that Jason was hovering over the baby.
Before you can say, "He's evil!" Horatio and Sevilla are out at the Caplin place, where a remarkably resilient Jason Caplin is holding one of his children without attempting to kill her -- something of an aberration, given recent events. He then takes Horatio for an insurance adjustor ("they're pulling up the flooring in my house because it's rotted with blood"), and after he gives a spiel of woes that does not once mention any deep or profound sense of bereavement for the three children or wife who were slain, Horatio cuts him off with, "Blood is why we came today. We want to talk about your blood on your baby." Erin squeals, and after calming her down, Jason's all, "I bled on my baby? I got shot. I guess my blood must have been everywhere." Horatio explains that actually, it's the gravitational droplets of blood that were the problem, as "that proves that you were standing over him while you were bleeding." Long story short: Luke died protecting Max from Jason; after a standoff in which Jason correctly surmised that Luke wouldn't shoot him while he was holding Max, Luke managed to get Max and head toward the door, where Jason, unencumbered by anything so pesky as concern over Max's life, shot Luke in the back a few times.