Crono's party finds themselves back in the Prehistoric era locked out of the Time Gate leading to 12,000 B.C. The party members then remember seeing the emblem of the doors of Zeal Palace somewhere before. Back in 2300 A.D. So, riding the Dactyls, the party heads from the Lair Ruins back to the Time Gate at the Mystic Mountains.
Traveling via the pillars of light at The End of Time, they head for Proto Dome in 2300 A.D. Making their way through Lab 32, they enter the Sewer Access, which then leads them to Keeper's Dome. Unlocking the sealed doors with the pendant, the party discovers the time machine called Epoch. Belthasar, the Guru of Reason, had downloaded his memory into a Nu. The Nu explains to Crono's party how the Epoch works. They then take the Epoch through time to 12,000 B.C.