As if hearing my blog post from across time and space (to, you know, when the show actually aired last summer), "Day Two" hits all the sweet spots of building the scope of an on-going narrative while remaining fiercely devoted to character.
After all, the core characters have benefited the most from Torchwood's transition to mini-series format. Children of Earth has yet to fill a single frame with filler, and never is this more obvious than in the intimate scenes between characters. Whether it's Gwen telling Rhys about her pregnancy as they flee to London in a potato truck or Ianto meeting his sister in the park all dirtied up from the explosion at Torchwood-3, the show takes its time to put the characters in perspective. As an added bonus, Captain Jack Harkness, arguably the most developed character coming into Children of Earth, spends much of "Day Two" dead, regenerated and screaming, cemented, then regenerated again. By putting him in the back seat, we get to see Gwen kick ass with two pistols Lara Croft-style while Ianto slips about London seeking out Jack and Gwen and wondering why the heck the government would want to harm one of its greatest alien assets when an alien threat is coming to Earth "tomorrow" (so say the children of earth). I also dig how Rhys and Gwen are no longer at each other's throats -- it was the main complaint I had about season two's handling of bringing Rhys into the fold, but while the couple still debate each other and question their collective next move, there is little of the hot-headed Rhys that was his defining characteristic in the series.
All of this is guided by an increasingly daunting narrative: gaseous-based life-forms are approaching Earth, using the children of the world (and an old dude, also on the run from the government) to communicate. Fortunately, we have Lois, an assistant to the guy in charge of whatever government branch this is (could it be UNIT? I don't really know, and the government branch is largely undefined beyond being "below the Prime Minister"), who aids Gwen and Rhys in locating Jack Harkness and realizing that Torchwood-3 was destroyed because someone with a lot of power doesn't trust them.
There's a lot to love so far. I can't say I was always a fan of Gwen in the series, but she grew on me as time went on; here, though, she's beyond multiple levels of badass. I don't know of many female characters who would engage in a shootout in a top-secret government facility with husband and fetus in tow. Ditto for Ianto, who like Jack is pushing beyond the traditional boundaries of gay characters by just being awesome: he totally saves Jack with that forklift, and he's smart in taking his sister's car and laptop after their meeting. Danger lurks at every corner for the people who love Torchwood-3 members, so every precaution must be taken.
Torchwood-3's members have been set up as the underdogs in the story, and that's fine, because underdogs make for awesome heroes. (See the list of critically-acclaimed cancelled television shows for reference -- and pour one out for new member Terriers.) "Day One" is a lot of plot setup and reintroduction to the world of the show; "Day Two" maintains the intensity but pulls back the pace so that we can see the characters we love transforming before our eyes from the two-dimensional players they were in the series to the three-dimensional human beings they are now. Gwen has developed a hardness, but maintains her love for Rhys and joy at having new life growing inside her; Ianto's love for Jack never clouds his judgment; and Jack is genuinely pissed at having been blown up, captured, chained, and cemented, a dramatic shift away from his usual flirtatious smile or hardened military stance of "doing what must be done for the safety of all humanity".
So the only question I have left is: what happens next? I'd say that's a sure sign of Children of Earth's success. The mini-series format has taken previously-developed characters and thrust them into an impossible, life-changing situation, yielding nothing but positive results for everybody.