Ash and co. continue their journey to Icirrus City, but to get there they have to cross over one humongous obstacle: Twist Mountain. There they meet a Pokémon Archaeologist named Ferris and his friend from childhood Sierria. Apparently Ferris has dug up a Cover Fossil, which is that of the Prototurtle Pokémon Tirtouga that lived long ago but now its extinct. But is Ferris the only party interested in the fossil? No, but not for long until Team Rocket came around to steal the fossil out of sight to revive Tirtouga here into the present so they would use its life energy to stabilize the space-time warp below Twist Mountain called the "Time Gate". And they would capture all of the Ancient Pokemons from the past!
In the flashbacks, it is revealed that Team Rocket's Fossil Restoration Machine is used with the power of chargestones from Chargestone Cave and using the same system data they steal from Professor Juniper and Dr. Fennel that revive the Archen from the Plume Fossil