World War Three is a satisfying, but not perfect conclusion to the story that began in Aliens of London. Not that there are any major defects; just some dodgy FX work when the actual aliens are revealed.
The green men in question - twist: they're big, despite what has been said in The X-Files - call themselves the Slitheen, and they are posing as the Prime Minister and his aides in order to declare a new war, which will end with Earth's destruction. The only person who can stop them is the Doctor, but since he is "trapped" in an emergency room with Rose and one Harriet Jones (Penelope Wilton), he's gonna need some help from Rose's boyfriend Mickey Smith (Noel Clarke), who doesn't really get along with the Time Lord to begin with.
This story has it all (minus the aforementioned perfection required from the special effects): suspense, action, great dialogue and interesting characters. Mickey gets to evolve significantly after being reduced to little more than an extra in earlier episodes, and Wilton's contribution leaves the door open for further developments later on. More than anything, though, the episode's real triumph is Eccleston, who has his go at another Doctor Who tradition: saying ridiculous names with rock-hard conviction. In this case, he adds his typical grin when he identifies the Slitheen's home planet, Raxacoricofallapatorious (a sci-fi tongue-twister), a rare case in which something that stupid ends up adding to the protagonist's charisma.