Tally feels like the oldest ugly in her dorm and is all alone now that she fought with Shay. And she's got one whole week to go before they turn her pretty. But everything will be better once she's pretty.
Then Shay shows up with equipment for one last, big trick: they're going to run away to the hidden town where David lives—the Smoke—and never get turned pretty.
See, a bunch of Shay's old friends ran away instead of being turned pretty, but Shay chickened out. But now that Tally told her that she had to grow up, Shay is ready to run away. (Which must be a big surprise to Tally, who basically meant almost the opposite when she said that they had to grow up.)
Even though Shay has awesome, solar-powered hoverboards, Tally doesn't want to go to a town where everyone is an ugly.
So Shay gives Tally some instructions on how to find the town—but it's in a code that only Tally should be able to understand.