An Enemy of Fate

All five of the primary Fringe Division members face-off against the invading Observers in a complex and extraordinary battle to manipulate the fate of mankind by altering future events and deflecting history away from the path to total oppression.

Synopsis provided by the Fandom Wiki, Fringepedia, and Wikipedia.

• All 12 Observers from the scientific team started to develop emotions by being around pre-Observer humans and they agreed to speak nothing of it.
• The plan to defeat the Observers: an initiating reactor will provide a surge of energy to start up the device. The device will create a singularity, a small wormhole. The radioactive power from the red rocks will sustain the device's energy. The beacons stabilize each end of the wormhole, one in 2036 and one in 2167. The large electromagnet is a launch platform, launching the beacon into the wormhole at the exact right angle to reach its destination. The scrolls of physics are to set and prepare the cylinders. Walter has to accompany Michael to guide him to the scientists in Norway.
• Walter knew that the universe would delete him and Michael at the point of the invasion in 2015. He made his goodbye tape to Peter sometime between 2012 (season 4 finale) and the invasion, when he and September were working on the plan. So when Walter disappeared the tape would still exist since it was made pre-invasion.
• Timestamp: 17:05
  December is dead, hung by Loyalists.
• Olivia asked Michael what to do next when they realized they couldn't trigger the device, and Michael put his finger to his lips as if to say 'stay calm, wait, and the solution will come to you.' Michael repeated this gesture after Olivia killed Windmark with her powers, staying calm and quiet.
• Timestamp: 23:06
  Gene has been in amber this whole time.
• Walter called Astrid 'Astrid'.
• After getting thrown to the ground by Windmark, Olivia saw Etta's bullet necklace on the ground. The feelings of fear and love inside her caused Olivia to summon all her strength and use the last of her Cortexiphan abilities to defeat Windmark.
• Timestamp: 40:04
  September is dead.
• Peter's last words to Walter were "I love you, dad."
• It would be a paradox for Walter to live the same years of his life onward from 2015 and also from 2167. He can't have two lives. So he was erased from 2015 at the moment of the would-be invasion.
• Time travel possibilities:

  • 2015 was a fixed point in time and so everything before it remained the same while everything from then on was different.
  • Walter stopped the Observers from being created, but knowing how instrumental Observers were in setting him on the path we saw all 5 seasons, he created special time travelers using Alistair Peck's time traveling device to go back in time and make sure all the same events still happened (like Marty McFly making sure key moments still played out so his parents would get married even though the specific details were different).
  • Walter still allowed Observers to be created, and possibly even a science team to go back and observe history, but they were like Michael in that they still had emotions in addition to higher intelligence. So they still went back, still distracted Walternate, etc. so seasons 1-4 still happened, but the invasion never happened because, having emotions, they never ruined their planet and therefore had no need to invade the past.
• Timestamp: 42:56
  One of the envelopes Peter sorted through said "Thank you for your support!", a direct message from the creators of Fringe to all the fans.