In the climactic season finale, everyone bands together in an effort to stop Samuel (Robert Knepper) from taking the lives of thousands. Peter (Milo Ventimiglia) joins forces with his most unexpected ally to save Emma (guest star Deanne Bray). Meanwhile, H.R.G.'s (Jack Coleman) life hangs in the balance as he and Claire (Hayden Panettiere) find themselves trapped underground with oxygen quickly running out. Elsewhere, Hiro (Masi Oka) starts to come to grips with the decisions he has made and is called into action to help stop a disaster. Greg Grunberg, Ali Larter, Zachary Quinto and James Kyson Lee also star. David H. Lawrence XVII, Harry Perry, Todd Stashwick also guest star.