I spoil here and there, so that's the warning from the geek.
The 2003 anime diverges from the manga more and more as things go on, but this volume is a blend of original material and new twists. This starts with the introduction of a character never to be found in the manga or "Brotherhood": a long-haired amnesiac boy who is mostly incredibly pale skinned... except for his right arm and left leg, both of which are tanned. In front of Ed, Al, Izumi, and Mason, the kid performs alchemy without a transmutation circle, but more than that, fuses a stone with his left fist, something no alchemist should be able to do (as Ed later explains, your body won't let it happen to flesh and blood).
To Ed's confusion, however, Izumi takes this boy in and becoming very doting towards him. But when Ed tries to question the boy on his own, the child transmutes himself so that his body becomes combined with his bed. Izumi arrives and helps the boy free himself, but is angry with Ed and Al for confronting the kid. Meanwhile, back in Central, Sciezska accuses Mustang of ignoring the murder of Hughes for the sake of his career, while a new character is introduced as Hughes's replacement in the investigations department... one Lt Colonel Frank Archer. (Yep, another character who was created for the 2003 anime alone). And Archer is interested in doing whatever he must to advance his career, no matter who he can step on for his career to skyrocket.
Al is the first to suggest that the arm and leg that the mysterious boy has are somehow the limbs that Ed had taken from him all those years ago when they tried to resurrect their mom. This causes the boy to run in terror from Ed, but also brings his existence to Archer's attention. The idea of a possible homunculus practically makes Archer drool, and he forces Major Armstrong to capture the child.
The resulting chaos occurs over the next episode and a bit, and leads Izumi, the boy, Ed, Al, and Winry back to Yock Island, where the youngsters from Risembool learn that Izumi has more in common with the Elrics than the boys thought, but it also starts Ed and Al to realize that what they did four years earlier may have yet more pain for them. The final episode introduces another character who is exclusive to the 2003 anime: Dante, who taught Izumi alchemy years ago. But while heading to Dante's mansion in the forest, Greed, the homunculus who broke out of Lab 5 when Ed and Al fought the armoured guardians and three other homunculi, arrives too. And Greed wants Al.
The similarities and differences between the 2003 and 2009 animes are amazing, and all the voice actors do incredible work to pull you in and make you care. The art is also pretty good--a lot of it isn't that different in quality to current animation. Personally, I prefer "Brotherhood", as the verging storyline seems to me to create character dichotomy that I can't reconcile (Ed, for instance, is willing to consider killing people in the 2003 version, even though he starts the anime declaring he refuses to take a human life), but it's still pretty good stuff.
And even if you just want to compare it, 2003 version to 2009, that's pretty fun, too.