After handing The Unquiet Dead to Mark Gatiss, Russell T. Davies returns to write the first ever two-part episode of the new Doctor Who: London, an alien invasion and loads of humor.
The laughs come aplenty already in the teaser, when the Doctor arrives in London and tells Rose she can go home, since he set the TARDIS coordinates so that she was gone only twelve hours. Naturally, something went wrong, leading to our favorite Time Lord being accused by Jackie Tyler of kidnapping her daughter for a year. With the matter settled, a new inconvenience emerges: a spaceship has crash-landed in the Thames and the Prime Minister is mysteriously absent, which might have to do with the fact that his replacement is actually an alien in disguise.
Aliens of London works because it takes one of the original series' trademarks (something bad happening here and now, in broad daylight) and adapts it to 21st century paranoia and angst. It's sort of reminiscent of old stories like The Invasion (1968), which starred Patrick Troughton - the Second Doctor - and introduced the organization known as UNIT, which returns briefly in this episode (no interaction with the Doctor, though: Davies and BBC were adamant that no complicated references to the old mythology be made during the first season). Plus, it's funny as hell: when Rose finds out the Doctor (who got slapped by Jackie, by the way) is 900 years old, she remarks: "My mum was right. That is one hell of an age gap!".