The movie starts out with an unknown character fighting the bottom half of a monster and is killed. On Earth, Gohan (Great Saiyaman) and Videl (Great Saiyaman#2) are called to help a suicidal old man named Hoi. He jumps but they catch him and he tells them of a great hero trapped in a music box. No one can open the box and he tells them that they need Shenlong to open the box because there is a great threat coming to Earth. Gohan brings him back to the Capsule Corp. and Bulma looks at it and says that they must gather the dragonballs. They gather them up and summon the dragon. They wish to open the box and a figure playing a flute emerges from the box. He looks very much like the figure in the beginning of the movie. He tells them his name is Tapion and that by unleashing him, they brought upon a terrible evil upon themselves. Trunks likes him and adopts him as his bigger brother. Him and Goten gather food and return to Tapion's hide-out and gives him the food.
Later that night, the evil monster in the beginning of the movie attacks Earth and Gohan and Videl are assigned to defeat it. Gohan and Videl beat it up but cannot destroy it, all of a sudden they hear the same tune as before when Tapion was freed and the monster dissapears, Gohan sees Tapion in the distance playing his flute. Tapion goes back to his hide-away and falls asleep. Then, a void appears and the top half of the monster emerges. Tapion then gets his flute and plays the same tune when the bottom half of the monster attack the city.
The next night Trunks comes back gives more food. All of a sudden the old man who asked for Gohan's help knocks the flute out of Tapion's hands and Trunks gets it. The old man tells Trunks that Tapion is not a great hero, but Trunks doesn't believe him and gives it back. Trunks is about to leave when Tapion asks if he would like to have dinner with him. After dinner, he takes Trunks back to his house and tells Trunks about his homeworld and his little brother (Minosha, who was in fact the character in the beginning of the movie who was killed). Trunks falls asleep and is about to go back to his place when Bulma asks him to tell his story. He tells Bulma about his homeworld where a cult of black-magic warlocks had released their master Hildegarn and tells her that only his flute and sword are the only way to stop him. When he and his brother kept the monster at bay by playing the flute a priest cut Hildegarn in two. The top half was imprisoned in Tapion and the bottom half was imprisoned in his brother. Then they were sent to different ends of the galaxy in hope that the full might of Hildegarm would never be released. She tells Tapion that she could make something like the music box for Tapion and the next morning it was finished. Later on we see the old man (Hoi) and the bottom half absorbing the humans for power. Later at night Tapion wakes up and is confronted by the top half monster. The bottom half attacks and almost destroys the Capsule Corp. building when the top half is reformed with the bottom half and Hildegarn is reborn. Goku and the others fight the beast but he is too quick for them. Trunks and Goten do fusion and become third stage Gotenks. Gotenks fires a truck load of fireballs (Rapid Fire Death Missile Barrage) at Hildegram. After the smoke clears they find out Hildegram is in a cacoon so they fire a ki-blast to destroy him instead the cacoon breaks and a second form of Hildegram emerges. He then hits Gotenks and they de-fuse and go unconscious. He knocks everyone out. Then in the distance Tapion starts playing his flute. Hildegram tries to stop him but Tapion imprisons Hildegram. Trunks wakes up and goes to Tapion. He tells Trunks the only way to stop him is to kill him and gives Trunks his sword. He raises the sword to stike but doesn't have the courage to do it. Hildegarn is released and the flute breaks. Hildegarn then kills the old man and continues his rampage. Goku then wakes up and powers up to stage three Super Saiyan. Trunks then takes the sword and cuts off his tail. He is about to strike again but Goku tells him to back off. Goku gets hit by Hildegarn a few times and that this is his only chance to stop him. He then does a Dragonfist Explosion and turns into a dragon and blows a hole into Hildegram on his weak spot (the center, where he was cut open before) and destroys him. Peace returns to Earth. Bulma then sets the time machine to Tapion's timeline, but before he leaves he gives Trunks his magic sword.