Mohinder: The mad scientist isn't doing very well with his experiments. The attempt at giving an employee of Pinehearst powers has resulted in someone even more mutant than Mohinder himself. The reason for this issue, Arthur realizes, is that there's a third element - a catalyst - missing from the two sides of the formula. Later on, Suresh actually puts his guinea pig out of his misery by giving him a lethal injection.
Elle/Sylar: Yes, these two meet up again. Arthur actually places Sylar inside a room with a chained-up Elle. The goal? To teach Sylar empathy so he can steal other people's abilities without murdering them. After being shocked numerous times by Elle - who is none too pleased that Sylar killed her dad and who wants to die because her short-circuiting is causing so much pain - Sylar eventually gets through to Elle. He eases her pain, takes her ability (though she appears to still have it, as well) and these two get quite cozy with one another. Total love connection.
Nathan/Tracy: At Pinehearst, Nathan meets his father and is shocked that he's been alive this whole time. Arthur attempts to recruit his "favorite son" to be President of a world in which have the citizens are given abilities. Nathan flies to Primatech to think this over and to see if his mother is okay. Meanwhile, Tracy essentially offers her services to Arthur. She says she can help Nathan come aboard as long as Arthur "scratches her back."
Hiro/Ando: Sorry, we're trying to remain positive, but this was a completely asinine story line. Immediately after Arthur grabs Hiro's head in Africa (which was how we ended last episode), Arthur is distracted by the drawing of an eclipse on a rock. Hiro and Ando quickly escape to Japan, but there's a problem: Hiro thinks he's 10 years old. Instead of killing him or taking his power, apparently Arthur has made Hiro believe he's a child. The hero therefore acts that way, as an exasperated Ando tries to tell him about his abilities.
Eventually, Hiro and Ando end up in a comic book store, where the latter shows the former Ninth Wonders in order to prove that Hiro really can travel through time. This seems to attract Hiro's attention, as the comic book depicts all his adventures in the episode. The end of the comic book shows an eclipse, with the words underneath the drawing: "It's Coming." (We hope that refers to the end of such an horrific use of Hiro.)
Peter/Claire: These takes turns helping one another escape from Knox and Flint. They end up in Angela's room in Primatech, which will be explained right now...
Parkman: Parkman and Daphne visit Angela at Primatech. Parkman gets inside her head in order to see why she's in a coma. Once there, he sees Angela handcuffed to a chair, a taunting Arthur and a pair of Daphnes. The first actually stabs Parkman, but then the present-day Daphne is taken from the hospital room into this semi-real dream. She convinces Matt that she loves him and was only siding with Arthur because she didn't wanna return to her old life (it is yet to be explained what this entails).
Newly inspired, Parkman returns to the hospital room, where Angela has awoken. And hasn't missed a beat! With Nathan, Parkman, Daphne, Peter and Claire at her bedside, Angela says Arthur has figured out there's a catalyst for the formula; and it's hidden inside someone's blood. At this , Claire remembers that Sylar told her - when he was cutting her head open, of course - that she was special. We all know why.
Claire is the catalyst for the formula. Save the cheerleader... and you know the rest!
Source: TV Fanatic