>Story opens with a tale of Yamoshi
>It fades to Broly travelling the universe as a gentle nomad warrior, having been unfrozen a few years prior (after Frieza saga at least) and fighting in intergalactic wars
>Frieza returns and recruits Broly as his right hand soldier
>Eventually Broly learns of the surviving modern saiyans and grows curious enough to go to Earth without Frieza's approval
>Broly comes to Earth and senses that Goku has the energy of Yamoshi (due to the SSG ritual) and goes berserk
>Cue curbstomp battle with Broly eventually being defeated after Frieza arrives and lends a hand
>Frieza explains to Goku and Vegeta the history of the saiyans, what the ancient saiyans were like, how his race knew of them and the Super Saiyan legend etc, basically a huge exposition that reveals that while Yamoshi was the original Super Saiyan God, Broly was the original Super Saiyan a thousand years prior.
>Goku and Vegeta train (Frieza ends up joining in eventually after being goaded) with Broly returning after a few weeks having grown vastly more powerful after honing his berserker form and gaining a zenkai boost. The trio are overwhelmed but Goku and Vegeta inform Frieza of the time chamber with Frieza telling them to train. Frieza battles Broly solo in a horribly one-sided fight but gets over his fears of the original Super Saiyan and admirably holds his own with his newfound power. Eventually he is defeated after hours of fighting
>Goku and Vegeta return and battle Broly but he is still too powerful, having simply grown after defeating Frieza.
>Goku and Vegeta are forced to perform the Metemoran Fusion to become Gogeta Blue and are complete equals with Broly, they enter a beam clash with Broly ala Cell and are losing
>Frieza awakens and distracts Broly, causing Gogeta to overwhelm the Legendary Saiyan, saving the day.
>Post-credits, Broly is shown to be alive but no longer enraged having calmed down and it ends with him questioning why Goku has Yamoshi's spirit