For one of the last few episodes of the series, this episode was surprisingly full of fluff. Some plot stuff is advanced, but nowhere near as much as you'd think.
The lab releases another diclonius, the infamous "number 35", who is not only Kurama's daughter (Mariko), but the most powerful of the bunch as she has a lot more vectors, a lot more range, and a lot more control than the others. In fact, the lab isn't even sure that they can control her, given her innate desire to kill all humans. They decide to implement two strategies, the first is a sort of "diplomatic" approach as they bring in the woman who has been acting as a human contact and mentor. As the closest thing to a mother the girl has, they believe that Mariko will listen. If that doesn't work, there is a bomb implanted in her that can be used to kill her if necessary.
Mariko's big reveal is one of the major highlights of the episode as it is an odd mix of poignant and creepy. When the idea of talking to her was first brought up, I pegged the "mother" as doomed. However, the viewer is given the impression that it might work. It doesn't, of course, but it was an odd back and forth of expectations.
Mariko puts up a menacing front as she levitates and starts unleashing her vectors. Her badassery is undercut when one of her arms just seems to fall off. I wasn't even sure what had happened, but the impression I get is that a bomb was set off. I guess they have smaller bombs implanted for maiming purposes and a larger one for an ultimate failsafe? It wasn't really clear, but the point is that, through threats, the scientists are able to recruit Mariko to their cause and she agrees to kill both Nana and Lucy.
Back at the house, Kohta tries to press Nana for information. When she doesn't comply, he gives the impression that he is kicking her out as he says that there is a price for staying. It seems harsh, but then he reveals that he just wants her to help with the chores. It's a weird mood whiplash, but it would have been out of character for Kohta to just kick her out like that.
Mayu and Nana bond some more as the prior becomes something of a mentor to the latter. This, of course, leads to squicky fanservice as we see two young underage girls bathing together. Nana admits that she doesn't want to kill Nyuu, but fears that Lucy will emerge and if that happens, she would have no choice but to fight to the death.
During a dinner conversation, the topic of Kohta's deceased sister is brought up, causing Nyuu to panic and start apologizing for some inexplicable reason as she pleads with Kohta not to cry. She then runs off with a pair of scissors. At first, I assumed that she was going to cut her hair, but when we see her, she opens the scissors with both hands and holds the scissors to her throat. On the one hand, you know she's not going to kill herself, but still, damn dude. That's kind of a messed up thing to use as a red herring.
Sure enough, she cuts her hair so that she resembles Kohta's sister. The way the scene plays out, I thought that it was going to be revealed that she was Kohta's sister and that she never really died. However, the characters come to the conclusion that Nyuu is just desperate to make Kohta happy and that she was putting on airs. It was a bit odd, but it was still a fairly effective scene overall.
As the episode ends, Nana goes to confront Mariko. It doesn't go well. Nana takes a second to come up with a strategy, only to have Mariko rip off her artificial limbs with little trouble. It looks like Nana is rather outclassed at this point, but tables have turned before so we'll just have to wait and see how that fight pans out. On top of that, there are only two episodes remaining, so it will be interesting to see how everything wraps up.