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This article contains major spoilers.
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Shortly after arriving at the Bureau of Genetic Welfare, Tris Prior [formerly called Beatrice Prior] discovers that the Memory Serum was one of multiple serums entrusted to the five factions of the city, the Memory Serum had been given to the officials of Abnegation. It is revealed that David, the head of the Bureau, uses the stores of serum at his disposal to 'reset' experiment cities that had gotten out of hand, without having to shut them down. Tris disagrees with the use of the serum to 'reset' cities, as she believes it is unfair to erase people of their identities and relationships.
When tensions between Evelyn Johnson and The Allegiant resistance reaches a certain point, David believes it is in the best interest of the experiment program to reset those inside the city, to reestablish peace, however his ulterior motive is not to spare lives, rather to convince the higher authorities to continue funding the program. Tris comes up with a plan to use the Bureau's stores of Memory Serum to reset the memories of those within the compound to cleanse them of the propaganda that convinced them that 'genetically damaged' (GDs) people were inferior to the 'genetically pure' (GPs).
All of those involved in the plan had themselves inoculated against the effects of the serum, however Tobias Eaton notes that Peter Hayes does not inject himself, instead pretending to do so by holding the needle against his skin, pressing down on the plunger and wiping away the liquid as it drips down his neck.
Once Tris is inside the Weapon's Lab, having swapped places with Caleb Prior, she is held at gunpoint by David, who believes that she is there to steal the serums and shut down the experiment cities, unaware that she is simply attempting to give the 'genetically damaged' equal rights by erasing the memories of those in the compound. Tris distracts David by discussing her mother, whom he was once in love with, and uses the distraction to close the distance between herself and the serum. She lunges for the container, being shot three times in the process, inputs the code she memorized from Caleb and Matthew and releases the aerosol into the compound, thus erasing the memories of all those in the compound who had not been previously inoculated against the serum's effects. (there are some details missing in this part. I suggest reading the actual books, since half of you reading this probably have to do a report and you're trying taking the easy way out)
The other part of the plan has Tobias return to the city with Christina, Peter and Amar, originally to bring Zeke and his mother, Hana back to the compound to say their final goodbyes to Uriah; however Tobias also has the plan to seek out either his mother or father and use the serum on them. After Christina slashes the truck tires as per the plan, Tobias convinces Amar that he will go searching for another truck, however Peter blackmails him into allowing him to come along. Peter reveals that he wishes to use the serum on himself as he is sick of hurting others without remorse and wondering what is wrong with himself, and after the two brawl briefly, Tobias agrees that after everything is done, he will assist Peter in wiping his memory. Tobias and Peter choose to seek out Evelyn, hoping that using the serum on her will cause the city to return to peace and allow Tobias to have his mother return to him. Evelyn refuses the serum, instead relinquishing her power over the city and negotiating a peace treaty with Johanna Reyes and Marcus Eaton.
No Future UsagesEdit
Having no further use for the serum, Tobias gives the vial to Peter, And asks him what Peter would like Tobias to tell him after he consumes the serum, Peter says he does not want any memories of his past life. Peter consumes the memory serum and quickly becomes disoriented and has to be reminded eleven times what his name is. It is later revealed that Peter has moved to Milwaukee and is working in an office job. While Peter was described to have changed significantly after ingesting the serum, some of the 'sharper, harsher aspects' of his personality were described to have returned after the haze of the serum wore off.
Some time soon after Tris' death, Tobias returns to his home in the Abnegation sector of the city and shaves his hair in typical Abnegation fashion. Afterwards, he brings out a vial of memory serum that he intends to use to erase the painful memories he has of Tris. He is stopped by Christina, who claims that he is taking the cowardly way out and will turn into someone that Tris would hate. Tobias becomes quickly enraged by this, threatening Christina, who manages to quell his anger and convince him not to wipe his memory after Christina once again blames him of cowardice for trying to attack her. Then, Tobias ( Four) goes to scatter Tris's ashes that he kept since she died.
The Memory Serum can be administered by intravenous injection, ingestion or viral contraction and subsequently erases a person's memories. It targets specific types of memories - memories that are explicit within the brain, such as names, faces, learned knowledge and sense of self. Affected individuals do not, however, have their implicit memories erased, such as how to walk, talk or ride a bike, thus allowing them to start their life anew. It has been used in the past to "reset" certain failure experiments.
In the first few days after the serum is administered, the affected individual is generally disoriented and their minds are considerably more pliable than usual. This allows unaffected individuals to give them new identities, plant false memories and thus enable them to assume a new life without feelings of displacement.
The memory serum comes with a counterpart inoculation serum that renders an individual immune to the memory erasing effects of the memory serum in any form, whether it by injection, ingestion or inhalation of its aerosol. Tris, Tobias and the other conspirators inoculate themselves so they will not be affected by the serum, as are Zeke, Hana, Nita and Christina's family. The Memory Serum, also known as the Abnegation Serum, erases someone's memories, erasing the important knowledge such as their name, place of birth, etc. though leaving the basic knowledge untouched, such as how to talk, walk, tie your shoelaces, etc.
It is released into the air of the Bureau of Genetic Welfare and reversed by Tris (Beatrice) Prior, in order to protect their home city, Chicago. According to Matthew, it can spread like a virus among people, and has been used a few generations before Tris.