The movie begins with a young boy on a distant planet frantically searching his current area, sword at the ready. Suddenly a loud noise alerts him that the thing he was searching for is behind him. However, it is too late to react as the young boy is smashed under the foot of a giant creature. Off in the distance, a villainous laugh can be heard.The movie then cuts to Earth as a bank robbery is happening. The bank robbers are getting away when Great Saiyaman, Gohan’s superhero alter ego, stops the car with his foot and smashes the front end flinging the felons out the car to confront Great Saiyaman II, Videl’s superhero alias (she is called the Great Saiyawoman in the English dub).
They apprehend the felons and turn them in to the police, all the while being spied on by some strange old man in the shadows. A short while later, they are summoned again from their school to rescue an old man who wants to commit suicide. When Gohan and Videl arrive on the scene, they discover that he is the same man who had been spying on them. After rescuing him, the old man introduces himself as Hoi and tells the two about the legendary warrior Tapion and that he needs their help in releasing him from the music box he is being held in, as the Earth would be endangered if he were not released. Gohan tries to open the box with his massive strength but to no avail. The old man tells them he knows of the Dragon Balls, and advises the couple to use them to open the box. They agree to bring the box to Bulma’s home first to see if there was another way to open the mysterious box, not knowing that Hoi had other plans on his mind.
They arrive at Bulma’s house with Goku, Goten, and Krillin. After analyzing the box, Goku tries his hand at opening the box but he cannot open it either. So it is decided to gather the Dragon Balls and summon Shenron to see if he can open it. After summoning Shenron, Tapion is released and the music box shatters into pieces. Angry at the fact that he was released, Tapion demands that he be resealed in the box, impossible due to the damage done by his emergence. A furious Tapion storms off, determined to stay alone in order to hide the fact he was holding the upper half of the creature called the Hirudegarn in his body. He isolates himself in a dilapidated building. The warrior is visited regularly by Trunks, who has taken a liking to him and brings him food, but Tapion refuses to eat and rebuffs Trunks’ attempts to bond with him. Trunks only becomes more determined to crack the loner’s icy exterior, as he sees in Tapion what Goten has in Gohan, something he never had (and presumably lost when Gohan started dating Videl): an older brother.
Shortly after Tapion is released, Hirudegarn’s lower half starts to wreck West City. Gohan and Videl arrive at the scene to investigate, but shortly after they arrive, they are almost swept away by the monster’s tail. They execute their Great Saiyaman poses, however Hirudegarn’s legs are not impressed; he throws the two heroes to opposite sides of the street. Gohan, with Videl’s support, immediately counter-attacks. After the short battle, Videl thinks Gohan has won, but Gohan says it isn’t over yet. Tapion comes to aid Gohan and Videl by playing a magical tune on his ocarina, weakening Hirudegarn’s bottom half, causing it to vanish. Leaving the battlefield, Tapion comes to believe that Hoy has already killed Minosha, Tapion’s younger brother.
Later on, while Trunks is again visiting Tapion, Hoy attempts to steal the ocarina from Tapion. Hoy is confronted by Trunks. The devious Hoy tells Trunks that Tapion is the true monster. The young Saiyan doesn’t believe him, and instead gives the instrument back to Tapion. Tapion then befriends Trunks, and asks him to dine with him. Tapion is invited by Trunks to stay at Capsule Corporation. While there, Tapion explains the whole story to Bulma.
Tapion starts his story beginning thousands of years ago. On Konack, a planet in the southern galaxy of the universe, a group of black magicians, the Kash-phar, use a spell on a statue to bring the monster it depicted to life. The monster, Hirudegarn, goes on a rampage and kills everyone who tries to get in his way, devastating the planet. Two young heroes, Tapion and his brother Minosha, manage to freeze the monster in his tracks by each playing their divine ocarina, giving a priest the opportunity to cut the monster in half with a divine sword. Since Hirudegarn can’t be killed at the time, his upper-half is sealed in Tapion and the other half in Minosha. However, fearing that the monster might reappear someday, the brothers are separated, each sealed in a music box, and shot into opposite ends of the galaxy. The music box holding Tapion lands on Earth. Soon after, Hoy, one of the Kash-phar, becomes determined to resurrect the monster to rule the universe. He releases Hirudegarn’s lower half on a distant planet and easily disposes of Minosha, with the monster crushed the young warrior underfoot. Now on Earth, Hoy plans to unite the Hirudegarn’s body and release the creature on Earth.
After hearing Tapion’s story, Bulma decides to create a room based on the broken music box design and the music the ocarina produces so that Tapion can finally rest. Tapion tries sleeping inside the new chamber, but he has a terrible nightmare that causes him to destroy the chamber. At the same time, Hirudegarn’s lower half is once again wreaking havoc, as he draws many men and women into his tail to devour their life energy. The next morning Goku, Gohan, Goten, and Videl arrive at Bulma’s house to inspect the destroyed chamber. Tapion appears suddenly, warning everyone the lower half of the creature is on the loose. To stop the creature from uniting its body he asks the group to kill him, in turn killing the creature’s upper half. Then however Hoy and the lower half of the Hirudegarn appear to stop anyone from killing Tapion. When Tapion is about to play the tone from the ocarina, the power of the lower half being so close to Tapion causes the upper half in Tapion’s body to react and release itself. Hoy succeeds in his plans, and with the monster back together, he believes nothing can stop him. A battle begins.
Goten transforms into his Super Saiyan form, Goku transforms into his Super Saiyan 2 form, and Gohan powers up to his Ultimate Gohan form to do battle. They launch their counterattack, but are quickly beaten since the monster can easily dodge by teleporting and countering their attacks. Gohan is grabbed and just before he is about to be squeezed into a pulp by Hirudegarn, Vegeta saves him by firing a ki blast at Hirudegarn’s arm and proceeds to fight. He transforms into a Super Saiyan 2. After a brief fight Vegeta is thrown into a nearby building but is knocked out after he saves the people in the building from Hirudegarn’s flame attack by using all his powers to create a barrier strong enough to withstand the attack. Goku comes in to help but still is unable to beat the monster and is knocked out. Goten and Trunks become Gotenks with the Fusion Dance. Gotenks, now a Super Saiyan 3, seems to be enough to fight the monster and fires his Die Die Missile Barrage attack at the monster. Hirudegarn is believed to be dead or at least neutralized but the attack only causes him to evolve into his second, more powerful form. Now in his new insect-like form, Hirudegarn mows Gotenks down to the ground and splits Gotenks back into Goten and Trunks. Gohan and Videl are then thrown into a building and Goku into some rubble. However, Goku isn’t out and keeps on fighting with all his might.
Just then, Tapion plays the ocarina again and manages to seal Hirudegarn inside himself. He then asks Trunks to kill him with his own sword, which will destroy the trapped monster. Trunks hesitates at first, but is about to do as requested when Hirudegarn breaks free. His ocarina is destroyed in the process. Hoy (who’s been watching the whole fight) mocks Tapion for being so foolish, but is killed as Hirudegarn steps on and crushes him. Just then Goku reappears in his powerful Super Saiyan 3 form and a battle begins between Goku and the Hirudegarn. Nevertheless, Trunks tries to fight Hirudegarn too with Tapion’s sword and manages to cut off the monster’s tail, but Goku stops Trunks. Gohan yells at Goku about Hirudegarn’s weakness which Goku has already seen and uses his Dragon Fist to pierce through Hirudegarn, obliterating the monster. With the monster gone for good, Tapion uses Bulma’s time machine to go back to his own planet, after giving his sword to Trunks.