When Phineas and Ferb's ancestors, Collin and Chris were alive, they were explorers. They kept trying to find the ancient Reverstone, which could reverse any day of their lives. The day before they retire, they find it, but there is an avalanche in the cave where they did. They decide to use the Reverstone, but it is on the other side. Back in the present, Phineas and Ferb finally get busted by Candace and decide to go find the Reverstone themselves to make it so they didn't get busted, or build anything that day.