Anime Movie, It shows a short story of the Ultimate Saiyan God that once ruled the so called Planet Vegeta until the god was stopped by the Kai God Sai who vanquished the god into a container that is now located in the Kai Vault. The Ultimate Saiyan God Spinak destroyed planets for fun and for landscape, once he did so he would then take a saiyan sacrifice.
Goku and Vegeta come into this story as their great ancestors were killed by Spinak and a special group of saiyans sneeked into the vault and opened the container using Elder Kai's key.
As soon as Spinak was released, havok started raging in the universe as the enraged god when berserk. He destroyed the Kai's planet and leaves which alerts Goku, Goku then travels to the ruins of the kai planet finding Kibito Kai and Old Kai fighting for their lives.
They warn Goku that he is just too powerful for anyone to handle, especially a Saiyan. Goku ignores and rushes to the Legendary God. Goku turns Super Saiyan 3 and strikes the god taking barely any damage to Spinak leaving Spinak to do a deadly kick which pummels Goku onto another mini astroid and then escapes using instant transmition just in time before he gets fried.
Goku travels back to earth to Vegeta as he asks for his help to defeat Spinak. Vegeta at first refuses to fuse with Goku but soon changes his mind.
Goku and Vegeta became Gogeta and went after Spinak but after a small battle it was just too much for him to handle and was left knocked out and on the edge of death. The Legendary Saiyan God has the power to turn into a Super Saiyan Form that was beyond belief and Goku could tell that it was stronger than the Saiyan God form he transformed into.
The group of saiyans that rescue Spinak return and Spinak gives them a wish and they wished for ultimate power. Spinak accepts but instead scorches them leaving them in ashes.
Gogeta returns to Dende's Lookout and speaks with the old kai about a way of defeaten Spinak, Old Kai then tells the story of Spinak and that Bills once fought Spinak and lost. It would take the fusion between two gods to have a chance of defeating Spinak.
Goku and Vegeta then learn that with Bills power, they can both transform into Super Saiyan God's and fuse. Bills arrives on Dende's Lookout and gives power to the both of them which then the 2 Saiyan Gods fuse into Gogeta God.
Gogeta God challenges Spinak and the fight goes on for a long time till Gogeta (God form) kills Spinak.