Creating a successful spin-off of a popular television series – a precarious balancing act between pleasing fans of the original and building a distinct brand – is a notoriously hit and miss affair. For every Star Trek: The Next Generation there’s a Joey. The anagrammatical Torchwood is Russell T. Davies’s attempt to explore more adult themes in the Doctor Who universe starring one of The Doctor’s most popular companions, Captain Jack Harkness (John Barrowman), and his crack team of alien investigators. The first two series made for enjoyable enough watching – if lacking in plot quality next to the parent programme – but they always left me feeling uncomfortable. For one, the exploration of adult themes through the lens of essentially childish story motifs never really convinced me. Nor, at the end of the day, could I buy into Cardiff as the centre of earth-shattering paranormal events. Perhaps it’s my limitation as a viewer, but if I’m going to accept a demon-spitting rift in time and space, I think it should be somewhere other than Wales.

Thankfully, as Torchwood’s third (mini) series, Children of Earth, kicks into blistering action, the location soon shifts to London for a grand world-encompassing plot to rival the epic scale of Doctor Who. This time around, the writers chose to produce five hour-long serialised episodes and it has clearly paid off. The format of the first two series, thirteen (largely) standalone episodes, was not always conducive to engagement with the heavier themes. By the time I’d started to buy each episode on its own merits and the ideas it was trying to sell me, it was over. The tightly integrated plot arc of Children of Earth, however, is much more effective. It convincingly builds character relationships, sympathy and emotion without losing sight of telling an entertaining story that will keep viewers on the edge of their seats (if not behind the sofa) for most of the five hours.

With creepy children speaking as one (reminiscent of Village of the Damned) warning of an impending threat, a powerful and largely unseen alien menace called The 456 with a suitably evil agenda, a government cover-up, a sinister ministerial plot headed by Peter Capaldi, who looks like he was born to play a civil servant, and a whole heap of morally grey choices mixed in with a bit of black humour, there is plenty to keep audiences engaged, even if the writers are hardly breaking new ground in the genre. Whilst the 456 do not actually appear until the third episode, their arrival is built up subtly and government assassination squads do a good job of filling in as early adversaries. Building convincing threat in science fiction, however, can be as tricky as creating a successful spin-off.

There seem to be three main schools on how to do this. On the one hand, there’s the Star Trek method. Fill an episode with ‘redshirts’ whose only purpose is to die so that when the all-powerful enemy arrives and the disruptors start flying, they’re there to take the fall whilst the main characters miraculously survive. On the other hand, there’s the Star Wars method, what’s known as the Stormtrooper Effect. All the villains are impossibly poor shots and the more enemies the hero is up against, the less likely he or she is to get hit. And then there’s Captain Jack Harkness, an immortal superhero who dies an unconscionable number of times, but always comes back to life. Indeed, in Children of Earth he dies so many times it is almost comical. No one reacts with surprise when he takes a bullet to the head, or has a bomb explode in his stomach, or gets buried in concrete, and lives to tell the tale five minutes later. One character even remarks in episode four, “a man who cannot die has nothing to fear.” Unfortunately, this rather diminishes the sense of fear we as viewers can feel for our hero, which detracts somewhat from the suspense.

Jack’s physical invulnerability, however, is compensated for by his emotional vulnerability. This is satisfactorily portrayed through the introduction of his daughter (Lucy Cohu) who appears older than her ageless father, and through his relationship with his teammates Gwen Cooper (Eve Myles) and in particular Ianto Jones (Gareth David-Lloyd). I have never been particularly impressed by the character of Ianto. Perhaps it is due to Russell T. Davies’s frustrating lack of creativity when it comes to naming his characters – there are far too many Joneses – but I have always found him to be a bit of a cardboard cut-out. His homosexual relationship with Jack, however, is well developed and emotionally believable without slipping into stereotype. And the occasional challenges to homophobic prejudices are conveyed with an appropriately subtle power without ever being overbearing so that the overall impression left is that the relationship between Jack and Ianto is as natural and everyday and caring as the relationship between Gwen and her husband (Kai Owen). This is not always an easy thing to do in fiction and many writers have failed to do what Russell T. Davies has done so well.

At the end of the day, though, fiction as a vehicle for ideas, no matter how powerful or important, is only as successful as the story it is telling. Does Children of Earth succeed? It’s no Doctor Who. But it is fun, funny, entertaining and engaging and I never felt bored or wished that I had chosen to spend my hour that day doing something else. In that regard, it does everything good television should. Whether Torchwood will survive for a fourth series, given the events of these episodes, remains to be seen. But after watching Children of Earth, I hope it does.