"To Win By a Tail" (悟空のシッポ Gokū no Shippo, lit. "Goku’s Tail") is the one hundred seventh chapter of the Dragon Ball manga.
After Goku uses the Afterimage Technique to dodge the masked fighter's Kamehameha, the masked fighter is in disbelief, but notes that it restricts Goku's maneuverability. Just as the masked fighter is about to take advantage of the situation by firing a Kamehameha at Goku, he notices that Goku is about to fire a Kamehameha of his own, which surprises the fighter. Goku fires his Kamehameha, hitting the fighter. He demands that the fighter give up before he finishes him off himself, when the fighter reaches out and grabs Goku by the tail, weakening him. Yamcha explains that Goku loses his power when his tail is grabbed, and wonders how the fighter recognized Goku's tail as his weakness. Master Roshi realizes who the fighter really is, just as the fighter slams Goku into the ground by his tail.
Meanwhile, Emperor Pilaf, who is watching the battle from a screen, overhears that Goku's weakness is his tail and is overjoyed. He reveals that he has one of the seven Dragon Balls in a special case which supposedly cloaks it on the Dragon Radar, preventing Goku from gathering all seven. Pilaf, Shu, and Mai then leave to seek out Goku and steal what Dragon Balls he has.
Goku refuses to surrender, and Roshi reveals that the masked fighter is actually Gohan, the late adoptive grandfather of Goku. As Grandpa Gohan swings Goku around by the tail and slamming him into the ground, Goku's tail rips off.