The Doctor and UNIT investigate Ruby’s past. But as the Time Window reveals horrifying secrets from Christmas Eve, the mysterious Triad Technology unleash the greatest evil of all.

We’ve been working up to this episode since the mystery of Ruby Sunday was introduced last Christmas, and while a few things are becoming clearer, it’s hard to fully appraise the strength of this episode without seeing what happens in the second part of this two-episode finale.

We jump straight into the story with the Doctor and Ruby arriving at UNIT HQ, meeting up with old favourites Kate Lethbridge-Stewart and Rose Noble as well as some new faces. Everyone collaborates to solve the mystery of just who the mysterious woman is that has been haunting our time travellers this season.

This is clarified right at the outset – she’s tech genius Susan Triad, or S Triad, or… obviously, TARDIS. Even nice-but-dim Colonel Ibrahim worked that out, and the bigger question is whether she’s actually the Doctor’s granddaughter Susan or someone (something) more sinister.

A trip back to Christmas Eve 2004 (Ruby’s birthday) via an old VHS tape might hold some answers, and provides a great opportunity for Ruby’s adopted mum and Mrs Flood to be pulled in to the mix. In fact, there’s an awful lot of plates spinning, and while it would be highly unlikely, someone starting with this episode would surely not have a clue what’s going on.

But let’s assume that all viewers have been here for the full journey, and let’s revel in the callbacks to previous episodes as it all starts falling in to place. It’s so good to see Mel (Bonnie Langford) back at the Doctor’s side, and Lenny Rush is a great new addition as scientific advisor Morris Gibbons.

And as the hour glass starts to empty, the grains of sand drawing us to the ending, there’s multiple areas of jeopardy to be resolved – oh, it’s great to have a big RTD cliffhanger for the first time since Christmas 2009.

Verdict: Exciting, big and satisfying (so far), let’s hope the final part sticks the landing. 8/10

Nick Joy