Saiyan Sized Secret (いまだ悟空! すべてを賭けた最後の大技, Ima da Goku! Subete o Kaketa Saigo no Owaza) is the thirty-first episode of the Vegeta Saga in the uncut Dragon Ball Z series. The episode first aired on December 20, 1989.
Risking his life, Goku charges up the Kaio-ken to a triple. The incredible power generated overloads the scouter that Bulma took from Raditz' body and begins to worry King Kai. With newfound strength, Goku ferociously attacks Vegeta. Unable to counter the onslaught, Vegeta is quickly overwhelmed.
At Kame House, Fortuneteller Baba works hard to conjure up an image on her crystal ball. Although they have no way of telling how well the battle is going for Goku, everyone is excited that Goku's energy is so high, but Master Roshi fears that Goku is pushing himself beyond his limits.
Vegeta continues to take damage from Goku, and grows furious that his own Saiyan blood has been drawn by a lower class warrior. Goku's body begins to have trouble enduring the strain imposed by the Kaioken x3, and he realizes he has to defeat Vegeta quickly. Fueled with anger, Vegeta decides to destroy Goku, along with the rest of the planet, by unleashing the Galick Gun attack. Goku counters with a Kaio-ken x3 powered Kamehameha, and when he decides it is not enough, raises himself to a Kaio-ken x4. Vegeta is hit point-blank by the beam and rocketed into the sky, leaving Goku exhausted from the surge of energy.
Yajirobe, who had been watching the entire battle in hiding, comes forward to congratulate Goku. A simple pat on the back turns out to be more than Goku can handle, and he screams in pain. Upon hearing from Goku that Vegeta is still alive and will return soon, Yajirobe again retreats into hiding. Freeing himself from the crest of the Kamehameha, an outraged Vegeta decides to transform. After failing to find the moon, he returns to Goku and reveals another way to transform. Vegeta tells Goku the origin of the Blutz Waves, reflected from the moon, and how they cause a reaction in the Saiyans. When thrown into the sky, the Blutz Waves are activated, allowing Saiyans to transform. It all seems to confuse Goku, until Vegeta creates a small, artificial moon with his own energy. With his tail gone, Goku is unaffected, but as Vegeta stares at it, he completes his terrible transformation into a Great Ape.
- Goku vs. Vegeta