Over the last few years, 'Doctor Who' has become something of an ensemble piece. Expanding from the doctor/companion dynamic of the Ninth Doctor and Rose Tyler and the Tenth Doctor and his companions, the show has seen fit to bring in more secondary characters who have appeared on more than one occasion, often joining the Doctor for the occasional trip on the TARDIS. Of course, the Doctor has had more than one companion at different points - just look at Amy Pond and Rory - but these have tended to be more consistent. Most companions had family members who were back home worried about them and therefore had to make the odd appearance but in the last few years the Doctor's friends have continued what he started, outside of the TARDIS.
One rather poignant episode saw the Tenth Doctor (David Tennant) reunited with many of his former companions, their relatives and friends, including Sarah Jane (Elisabeth Sladen) - who went on to have her own children's TV series 'The Sarah Jane Adventures' - and Captain Jack Harkness (John Barrowman) - who went on to star in 'Torchwood'. The idea was introduced that just because a companion had left the TARDIS and, by extension, the Doctor, they had not necessarily returned to their life before having met him.
This time around, with the Eleventh Doctor (Matt Smith), the unlikely trio of Jenny, Strax and Madame Vastra - also known as the Paternoster Gang - were introduced. First brought in to help save Amy Pond, the human Jenny (Catrin Stewart) and Silurian Vastra (Neve McIntosh) were breaking all the borders with an inter-species lesbian marriage and, with the bloodthirsty but hilarious Sontaran Strax (Dan Starkey) as part of the team, they certainly made for entertaining viewing.
The trio took centre stage in 'The Crimson Horror', conducting their own investigations which, in turn, led them to the Doctor and Clara. They then appeared again in the series finale, 'The Name of the Doctor', which saw the Doctor show just how important they all were to him by explaing to Clara that 'they cared for me during the dark times'.
After the Tennant era of 'Doctor Who', the bar was set and it would have been nearly impossible to match his level of success. The show has lasted for fifty years, though, thanks to its ability to regenerate, reimagine and bring new ideas to an old concept. It was a wise move, then, to expand the Doctor's universe and give him two companions at once, more River Song and three friends who don't often travel in the TARDIS with him but who do carry on his work protecting planet Earth. Many of the Doctor's friends and companions have come and gone in one or two episodes but there is something comforting and reassuring in having friends that just keep popping up - to the viewers and the Doctor himself.