This is a solid episode.
The premise is simple and effective. A creeper who openly inspects family homes in order to hide himself away so he can emerge from the shadows later on to terrorise the family. He's like a literal boogeyman hiding under your bed.
It's a good idea for a grounded episode as it's very easy to imagine this kind of method actually being successful. It doesn't take a lot to picture yourself being caught up in a horror like this.
Little girl - traumatised by what she's seen. The episode presents an interesting dilemma in regards to when it's appropriate to attempt to forcefully extract crucial information from a victim. One who is impressionable and could face serious psychological damage if made to relive the horrific experiences. An added factor that I thought was clever was the idea that the killer actually wants the little girl to be forced to relive the horror - I hadn't considered this angle and it makes him even more sinister and calculating.
The world has changed. Approaching the end of the Millennium, Frank and Catherine discuss the nature of things and conclude that it's not possible to return to more innocent times. Things have moved on, to a darker, sicker time and all one can do is try to shield themselves from the world's horrors. However, The illusion of safety and security is a big theme of the episode. This fits in well with the series as a whole which often gives a feeling of unstoppable evil encroaching upon a world that is powerless to protect itself.
Catherine Black is once again given a practical role due to the involvement of a traumatised child victim.
The opener is one of the best of the series thus far.