Sammy Oak is a ten-year-old Pokémon Trainer displaced forty years into the future from his own time. In his past, traveling near Arborville, he met a woman named Towa who spoke of the "Voice of the Forest" (Celebi) and rare Pokémon. Sammy defended Celebi from a poacher, causing Celebi to teleport them both forward in time.

Discovered unconscious in the woods by Ash Ketchum and friends in the present, Sammy awakens and learns from the elderly Towa and her granddaughter Diana about Celebi's plight. He joins Ash's group to find the missing Celebi. They encounter the Iron-Masked Marauder, a Team Rocket agent who captures Celebi using a corrupting Dark Ball. Sammy battles the Marauder's Scizor and Sneasel and helps guide the group to the Lake of Life to heal Celebi.

In the final battle, Sammy breaks the Marauder's control over Celebi by reminding it of their shared past joy. When Celebi collapses, Sammy carries it back to the Lake of Life. After the lake's waters fail, Sammy witnesses other Celebi from across time revive it. Celebi then returns Sammy to his original era, with Sammy promising Ash they will meet again.

As an adult, Professor Samuel Oak possesses vivid memories of these events, holding a childhood sketchbook depicting Celebi and Ash's Pikachu. He acknowledges this experience shaped his lifelong research into human-Pokémon relationships. His encounter with Ash directly inspired his inventions, including the Pokédex, conceived after Ash described such a device.

Returning to his time, Sammy pursued Pokémon training and research, eventually becoming a renowned Professor. He maintained a deliberate connection to Ash's family, befriending Ash's father and mentoring Ash's mother, Delia. He ensured his presence at Ash's birth and became Ash's mentor, fulfilling his promise of reunion. His experiences with Celebi and Ash fundamentally shaped his philosophy on friendship across time and his scientific focus on human-Pokémon bonds.