After the successful regeneration of Doctor Who in 2005, the showrunner Russell T Davies could do no wrong. The BBC was so thrilled with his teatime Time Lord that it was bound to commission anything he’d pitch next. Nobody guessed that would be Torchwood.

Torchwood (an anagram of Doctor Who, geddit?) was, unlike its parent show, strictly for adults. The show centred on the activities of the Torchwood team, who guarded a mysterious “rift” in space-time that gave aliens and time-travellers easy access to Cardiff’s Millennium Stadium. They were headed by the immortal, pansexual Captain Jack Harkness, a character played with flirty flamboyance by John Barrowman. One of the trademarks of the series would be the fluidity of most of the characters’ sexuality. Just to make sure viewers knew what they were in for, the second episode featured an amorous alien gas that fed by dissolving people in the moment of sexual ecstasy. People came and went in the same instant.

Facebook Twitter Pinterest Torch bearers ... Gareth David-Lloyd, John Barrowman and Eve Myles. Photograph: Steve Brown/BBC

Despite bonkers sex aliens, some viewers thought Torchwood took itself too seriously at first, with its stars brooding moodily on rooftops and battling giant, poorly rendered CGI bad guys. It wanted to show the stakes were just as high as on Doctor Who, but without leaving Cardiff the threat levels always felt limited. The second season adopted a much lighter tone and ran with it. It responded to the criticism levelled by some tabloids at Captain Jack’s sexual preferences by introducing a fellow time-traveller who liked not only men, women and aliens but was not averse to the charms of a poodle. Characters laughed, joked and, in the case of Captain Jack and the Torchwood tea-boy Ianto Jones, formed the cutest gay couple on television.

The wheels began to fall off the ubiquitous black SUVs in the third series. The final episode of season two featured the deaths of almost half of the main team. Filling that gap was always going to be tricky but the show was given no time to recover as it switched from a monster-of-the-week format to a miniseries with a single arc. Children of Earth revolved around aliens who wanted to harvest Earth’s children to make drugs. Threatening kids has to be the most hackneyed way of wringing emotion out of any show. The series also ended the relationship between Ianto and Jack in the way so many shows tend to despatch their gay characters: one of them was unceremoniously killed.

Over 10 ponderous episodes of mostly filler, the fourth season, Miracle Day, explored what it would be like if people suddenly became incapable of dying. Made in collaboration with the US channel Starz, the show mostly jettisoned its UK-centric view and the British cheekiness that had made it unique. American actors filled the gaps left by the character deaths of previous series but none made any lasting impact. Ending the series on a cliffhanger did not force the show producers to renew it but did leave viewers annoyed. For a show that dealt with themes of immortality, its death came as a sweet relief.