"Letters to Transit" is Fringe Season 4's 19th episode.  Set in 2036, the episode follows FBI Agents Etta Bishop and Simon Foster as they try to locate the original members of the Fringe Division.  Etta hires the services of a Native to track the Fringe Division team. On the night of their rendezvous, Native brings Etta to the location of one of the Fringe Division's members --- at the back of his van. 

There Etta finds the ‘amberized' body of Walter Bishop.  As it turns out Walter intentionally put himself in amber when the Observers started their takeover.  Etta goes to Simon Foster, an FD scientist to recover Walter from his amber state. They succeeded, but unfortunately Walter suffers from a brain damage in the process.  Etta knows that Walter's brain and memory are crucial in her quest to track down the rest of the Fringe Division Team so she has to take action and restore Walter's brain.  And so Etta and Simon seek the help of Nina Sharp.  At this point, it is revealed that Nina's involvement with the Observers is just a front because she is in fact a member of the resistance.  In order to help Etta, Simon and Walter, Nina reveals that sometime in the past, following orders from Walter, William Bell removed a piece of Walter's brain and she believes that this can be used to help Walter.  However, this brain piece can only be found within the old Massive Dynamic Facility inNew York's main island.

And so Etta, Simon and Walter penetrate the Massive Dynamic Facility's vault and successfully retrieve Walter's brain piece, but without raising an alarm and notifying Philip Broyle.

With Walter's brain piece in their possession, they finally set Walter's recuperation process.  After successfully mending Walter's brain, he starts his recount of the events.  Walter explains that by 2609, the Observers had finally devastated Earth and that the Observers traveled back in time to take control over Earth at an earlier point in history.  In the midst of their ‘tête-à-tête', the Massive Dynamic Facility forces catch up with them.  In order to escape, Walter assembles an anti-matter device which blasted the entire Facility into pieces.

After their escape, Walter leads Etta and Simon to the location of Peter Bishop, Astrid Farnsworth and William Bell, all of them ‘amberized'.   They start to free the three from the ambers but Broyles forces start to approach.   By the time they have freed Astrid, their machine start to malfunction, and to fully push out Peter from the amber, Simon sacrifices himself.  And with no time to completely save William, Walter severs William's hand before fleeing the scene. 

On board a train, it is revealed that Etta's real name is Henrietta, daughter of Peter and Olivia.  The episode ends with the reunion of Peter and Etta, father and daughter.