"Mystery City" is episode one of Gravity Falls: Mystery City.
Transcript[]
[Dipper and Mabel are biking home from high school in a hiking trail]
Dipper: Mr. Swartzenhighmer. What a wierd name!
Mabel: Especially for an English teacher.
[Dipper and Mabel both laugh.]
Mabel: Man...this road is really bumpy!
Dipper: No kidding.
Dipper's bike stops and he gets thrown onto the trail.
[Dipper stands up, wiping the dirt off of him. Mabel stops her bike.]
Mabel: You alright, Dipper?
Dipper: I'm fine.
[Dipper looks to the ground, where the dirt is swept off of an old encrypted sign that says, "MC this way"]
Dipper: Mabel, look! An encrypted sign...
Mabel: MC this way...what the Sally do you think it means?
Dipper: MC must be an abreviation..
[Dipper walks towards some pine trees. He shoves them aside, and sees a purple sign that reads, WELCOME TO MYSTERY CITY]
Dipper: Mystery City?
[Mabel walks past the sign following a gravel trail. She motions Dipper to come.]
Mabel: Come on, Dipper!