Life goes from routine to a roller-coaster ride overnight for artistic teenager Andi Mack. It all happens on the eve of her 13th birthday when free-spirited older sister Bex -- who has been traveling around the world -- returns home to make a revelation, which changes everything for the Mack family. It turns out that Bex isn't Andi's sister -- she is her mother -- and Celia, who Andi thought was her mom, is actually her grandmother. Learning this family secret sends Andi on an uncharted course of self-discovery. She's not alone on her journey, as best friends Cyrus and Buffy are also trying to figure out their places in the world.