Video : The Synchronicity Test!

Clemont has been wondering about the mysterious change that sometimes happens to Ash and Greninja during battle, and he wants to get to the bottom of it—with science! He challenges Greninja to battle his Luxray while “Clembot Mini” performs an analysis, but nothing unusual happens.
While Greninja is healing up, Alain and his Charizard stop by the same Pokémon Center and enthusiastically agree to the next battle! During this fierce competition, Ash and Greninja sync up, and Greninja transforms in a spectacular display of power—but after Ash has an intense and scary reaction, the battle is cut short. Still, he’s determined to get stronger alongside Greninja!

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S17E24 - Making Friends and Influencing Villains!