Operation: Don't Forget goes into effect as Amar and George agree to help Tobias, Christina, and Peter infiltrate the city. They inoculate themselves against the memory serum… all except Peter, who only pretends to (and ends up on the receiving end of tons of Internet hate from vaccination advocates).
Christina later approaches Tobias with a plan: he should erase one of his parents' memories, thereby making it easier for them to negotiate a peace treaty so that no one will die.
Tobias gives that idea a thumbs-up, and Christina agrees to snatch some memory serum. She also agrees to create a distraction later so that Amar and George won't follow them.