'Fringe' recap: Bathroom Breakup
Olivia's also having some difficulty concentrating on the case, what with having come around to remembering everything about her original timeline. Including, one assumes the Peter and Olivia Sexytimeline. Obviously this is unsettling news to Peter, who's been operating this whole time under the mistaken assumption that he's in the wrong universe with the wrong people. And a session with a suddenly calm Olivia under Walter's magic hair dryer doesn't improve Peter's mood, especially when Walter blames Peter for somehow projecting his memories into Olivia.
Meanwhile, they get a break in the case, suggesting that Sean may be a brother to one or more of the killers -- which is odd, because he was an only child. Except that his mother conceived him with IVF from an anonymous sperm donor, through a clinic that was being looked into by the very man killed early in the episode. Walter forms a theory that Sean and his "voices" are operating as a hive mind, like a colony of bees with a powerful protective instinct, while Peter and Olivia track down the fertility doctor in question at an assisted living facility. That gentleman calmly informs them that he messed with the embryos' DNA (all of which came from his own nasty loins, of course) back in the day, so now his science-demon seed are killing anyone who might make their story public. When Peter and Olivia go to check out a stash of the doc's data, they walk into an ambush, and the doctor himself is killed. Plus, Sean is cut out of the hive mind loop, which he takes pretty hard, even after being comforted by Astrid.