Brief Recap: The mission to retrieve Peter goes awry the moment Walter, Olivia and three of the Cortexiphan subjects arrived in the alternate universe. One Cortexiphan subject died upon arrival while the other two died using themselves as a distraction to prevent Olivia and Walter from being caught by the alternative Fringe team. By the eleventh hour, Olivia was captured. Fauxlivia took her place as she joined Peter, Walter back to our universe and William Bell disappeared in a massive bright light.
Spoiler Alert for those who haven't seen it.