Catherine tells Gil the foreign DNA on the bones is from Marissa's son, and Gil points out that there wasn't a second body in the grave and adds, "She never gave birth." "Not through her birth canal," Catherine points out. Gil muses, "That might explain the hunting knife we found." And then we flash to Walter giving Marissa a C-section without benefit of anesthesia or, for that matter, consent.
Back in the present, Catherine points out that the umbilical cord contains fetal DNA, and we go to the TMIcam time-lapse shot of the livid cord turning dark and desiccated over time. Gil muses, "Walter Darien never threw anything out. So where did the baby go?" Oh, there's plenty of places to explore at the Hostile Hostel. Just be sure to send someone with a strong stomach to do the looking.
Or maybe just talk to the wiry young man who's the father of the missing baby. He mumbles, "I never thought she was dead. I wondered where she was, what she was doing with her life, what my kid was like, why I didn't think to maybe talk to her family or an investigator to maybe see if I could answer the first three questions." Or maybe I'm projecting again. Catherine and Brass break the news that they're not sure if the boy was dead or alive. The guy tells the usual story: boy meets girl, boy impregnates girl, boy and girl decide to buck the odds on shotgun marriages between teenagers, boy and girl discover the odds are high for a reason. Marissa took off after a fight over the phone, and that's the last this young man had heard of her. Catherine looks empathetic -- no big surprise there.
It's up to Brass to ask actual questions, so he does: "You never reported her missing?" The guy replies, "I didn't think she was." I'm going to go out on a limb here and guess that this couple had problems communicating. Anyway, Brass cocks a skeptical eyebrow, and the guy explains, "I was nineteen! Barely out of high school." Catherine gets a little suspicious and comments, "Last thing you needed was a family." The guy continues, "She was in her ninth month when she split. I thought she'd cool off in a couple of days, come back, we would work things out. But then I got this." Catherine reads the letter he pulls out: "You're not who I thought you were. You don't want me. You don't want this baby. We don't want you. Marissa."
Meanwhile, Liam's sequencing the DNA from the hair found in the baseball cap, and Ronnie down in QD is busy explaining to Catherine how Marissa's signature on the letter was forged. Catherine bets that Walter did it, and Ronnie manages to speak for evidence, as opposed to inference and supposition, when he points out that she won't know for sure without an exemplar. Catherine remembers that CSI used to stand for Crime Scene Investigation and not, as one might suspect from this season, Crime Stories Imagined, and asks for the down low on the letter. It was printed on an inkjet printer, which doesn't square with any of Walter's dot-matrix numbers.