John Barrowman returns as Captain Jack Harkness, while Eve Myles is Gwen Taylor, the pair having spent the time since the end of the previous series getting over the deaths of their colleagues and now reunited, ironically, by an apparent global inability to die.
American co-stars add to the line-up, including Independence Day's Bill Pullman in what, three episodes in, seems to be significantly more than a walk-on cameo. However, we're yet to discover the role that his character, Oswald Danes, might play in the shake-out of the Miracle Day story arc.
Captain Jack sums the story up pretty well in the clip below - this is far from a Doctor Who-style space-and-aliens story. It is, if anything, similar in structure to the David Tennant/Catherine Tate episode Partners in Crime, which featured the Adipose, a creature produced from human fat thanks to a 'diet pill'. But it's less space-themed and much darker, with the human race effectively becoming a planet of zombies, unable to die whatever their injuries - yet still capable of feeling pain.
One of the best quirks of the story is that, just as everyone else on the planet is rendered immortal, the ever-living Captain Jack becomes mortal once again. It's a chance for the character to reassess his carefree style of living - and to think about short-term issues like friendship and love, the kinds of emotions most immortals have to put to one side (again a theme touched upon in Doctor Who and other 'immortal' stories like Highlander).
Critics have objected to the overly American style of the show - and in places, it does feel somewhat like the ill-fated Doctor Who TV movie which starred Paul McGann as the Time Lord - but there are plenty of scripted jokes highlighting the cultural rifts between the US and the UK (and Wales in particular) which help to keep things moving and provide light-hearted relief from the horrors of the main story arc.
Putting to one side the jokes about Americans thinking Wales is in England, this is still a traditional Torchwood production - and as horrific as things get, there is still plenty of humour to be found. That makes it all the more surprising when a tender moment - such as Jack's affection for Gwen, who is now not only married but also a mum - comes along.
Captain Jack is undeniably Torchwood's version of The Doctor - undying but faced with losing companion after companion. I've always thought of Doctor Who as a metaphor for the way human friendships develop and die in real life - often for no real reason. In Torchwood, particularly at the moment, there's an overt effort to make that a core element in the storyline, and it will be interesting to see how it progresses - will we see Captain Jack stride off into the sunset alone at the end of this series? It seems likely, in my opinion.
The iTunes links on this page are to the series download - if you're a fan, the extras that are bundled into that purchase, such as character profiles, are bound to be a welcome addition. If you're a newcomer, they could help to get you up to speed with what you've missed in previous series. New episodes will be queued and downloaded as soon as they are available - or you can use the links within iTunes to get them one by one, if you prefer.