The first two part story of "Nu Who" opens with Aliens of London. A ballsy episode that eschews keeping the aliens under wraps and instead makes the media response to such a event part of the story.
Heading back to present day London, aiming for just a few of hours after they left. A miscalculation with the Tardis leads to Rose (Billie Piper) and the Doctor (Christopher Eccleston) arriving almost a year later. Jackie (Camille Coduri) has reported her daughter missing and is naturally astounded when she shows up. Their reunion is cut short though, when an Alien ship crosses central London, crashes into Parliament Tower and lands in the Thames.
What I like about this episode of "Doctor Who" is that it's the first time that interaction with the "real world" happens. Usually, in most episodes, the exposure to the alien is limited to a few people who are either killed, or saved by the Doctor and keep their stories to themselves. This one, the invasion is on worldwide TV almost as it happens - and the Doctor has to brush up against the Army, U.N.I.T and the Government before getting to the centre of what's happening in this episode. There are a few other aspects of the show that are interesting. Penelope Wilton debuts as Harriet Jones, a politician who will feature a few times across the Russell T Davies era and Naoko Mori debuts as Sato - who will go on to feature in "Torchwood". It's a nicely plotted first half of a story that's both exciting and relatively clever.
It's let down, somewhat, by its alien threat though. The Slitheen are OK when they're in their natural form towards the end of the episode, even if their faces could have been a little scarier. But the repeated farting gag gets tiresome pretty quickly and the unzipping and transition effects don't really hold up that well. I appreciate it's a balancing act, in not scaring the children too badly, but the Slitheen could really have benefited from being slightly more scary overall.
It's a good episode though, with the second half to come.