Subtle hints to Fauxlivia and Walter tell us the emotional journey of these parents who lost their only son, Olivia retreats to a life of solitude that stuns her other Universe counterpart and, Walter retreats to the lab that costed him his only child.
The warning ensued by the man who wasn't there, throws both Fringe Divisions to join forces and fix the mess created by the collision of their Universes. The mess at large, being fixed by another kind of Division, an Observers one.
Using Lincoln as a gate between these breaches, it's Agent Lee the one who makes the connection between the case that killed his partner - Ironically, named after the grandfather of the friend he never met - and the phenomenon killing people. As Robert once told him, everything happens for a reason, even if that reason escapes him like the man in the mirror escapes them all. The closure Lee provides for his partner's family might be the start of the closure the Observers begin to provide for Peter's family.