All right then. I'll warn you. If you don't want a spoil do not invert the following sentences.
It's also bloody long. I'll do a rough rundown...if you have any concerns, I suggest that you make a conversation with me, but since it's too far-fetched, you might as well not.
Dante fought through his childhood with demons and grown to a certain age.
One day a member of the order shows up on his doorstep and tries to recruit him.(Which is Kat)
At first he ignores her but after being attacked by a "Behemoth" Kat informs that she knows Dantes family and secrets, and he decides to follow her.
At the orders HQ, he sees Vergil and he explains the demons clutches and Vilirity's side-effects.
In order to make a investigation he asks Dante as his brother, to help him out. Also Vergil says their parents are still alive somewhere. Dante agrees with the condition to let him see his parents.
By that time Mundus has impregnated Lilith, in order for the accension of Hell and the preparations for his heir.
(Missions include capturing Lilith and fighting Mundus in the middle of the story)
Finally it was revealed by the analasis that Vilirity's powers originally came from Vergils inventions, nano-machines that made him rich. The reason why Vergil invested to Mundus is that he thought his inventions will make a world without wars or discrimination by making the worlds populations mind closer to each other. (Which Mundus betrayed him, which explains why Vergil is fighting)
They manage to capture Lilith but Dante and Vergil got trapped when they were on a mission together. And got their angel/demon genes extracted. But due to help from the order, they got out of the facility.
Vergil tries to kill off Lilith, but Dante says no to the idea.
He also declares that the sort of"Hive-mind" control isn't just the way, with all of the sacrifices done at first.
Vergil sticks to his ideal, and has a temporary fight with Dante, but since it was true that his plan was used by Mundus in a fiendish way, he tears his ideal away and fights with his brother and manages to seal off mundus.(With Phineas's clues of Mundus weakness)
After the fight Phineas reveals his true face and takes the genes of the Brothers and become Sparda and they fight the last battle.