Buck shows up in episode 181 when Ash's group is ferrying over to Suzuran Island. While everyone else on board is heading to the Sinnoh League, Buck's got different plans—he's a treasure hunter making his way to a nearby island with his partner Baltoy, using the Pokémon's sensing abilities to track down valuables they've discovered before.
When Ash and the others help him out, Buck manages to locate the island's treasure and catches himself a powerful Claydol in the process.
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