Once the captain of the Jackal Squad and known as the ultimate mercenary, Infinite, once allied to Eggman, Infinite has gone on his own because now he thinks Eggman is weak. See Eggman wants to take over the world but Infinite's goal is to destroy the world. So now, we must work for Eggman to save the world so he can become its ruler.
Hopping into your battle trains, your goal is to chase down and destroy Infinite before he can access the Phantom Ruby. Infinite is insanely fast, however and will do anything and everything to dodge your attacks. It's no use attacking him out in the open so your job is to lure him into Eggman's hangar, corner him, and destroy him once and for all.