The best instalments of anything - be it television or film - make controversial decisions in their content likely to divide their viewers. That's what I like about Sarah Dollard's latest episode for Doctor Who, Thin Ice. Within those 45 minutes, you have The Doctor saying Jesus is black, Bill swearing, a kid dying.... All things you would expect the BBC wouldn't allow the show to do. Sarah Dollard certainly isn't afraid to upset her audience, given she - not Steven Moffat - was also the writer to kill Clara Oswald in 2015's Face The Raven. Clara may have been 'extracted' from her death in Hell Bent, but she still has to return to the moment she died eventually. Sarah Dollard's episode still remains Clara's main exit story.

In Thin Ice, The Doctor (Peter Capaldi) and Bill (Pearl Mackie) find themselves in Regency Era London during the time of the last frost fair. Under the frozen Thames lurks a chained sea creature belonging to Lord Sutcliffe (Nicholas Burns), who is using its poo as fuel to mine and sell to the local residents. Bill must make the ultimate decision: save the sea creature and potentially place others' lives in danger, or leave it trapped under the thames and deny the creature its freedom.

What's interesting about the creature is that it isn't alien in origin. It's just an ordinary sea creature that hadn't been discovered by anyone other than Lord Sutcliffe before this episode, like the mythical Loch Ness Monster (or the Skarasen, as those of us who are Whovians know it to be). It makes a refreshing change for the new series to deviate from the alien threat angle; the classic series of the show had done this more frequently with the introduction of species such as the Silurians and the Sea Devils. The last time the new series had introduced something from Earth as a threat without any alien interference was In The Forest Of The Night with regular trees, and even then they weren't a new species of tree as this is a new species of fish.

This isn't the episode's only earthly threat either. Nicholas Burns may not feature a great deal as Lord Sutcliffe but in the short space of time he does appear in he instantly makes an impression. Sutcliffe is the kind of villain you love to hate, and Sarah Dollard writes him as such a despicable human being that you punch the air when The Doctor punches him for making a racist remark. If only he would do the same for Donald Trump or Nigel Farage.

Those who didn't like the slow pace of the previous two episodes will be pleased to know that this episode has a much faster pace. With its strong focus on investigation and exploration it feels like a Jules Verne story, especially 20,000 Leagues Under The Sea. The bigger supporting cast give a more communal sense of atmosphere than the wonderfully lonely and mysterious one of Smile; this is a completely different episode of the show, and offers the kind of variety that is expected from Doctor Who.

The faster pace is definitely welcome, as it feels like since the previous two episodes we have really got to know who Bill is as a person. This episode is more an exploration of The Doctor's questionable motives and the darker aspects of travelling with the Time Lord; for example, after the child's death Bill asks The Doctor 'Have you ever killed anyone?'. The Doctor tells her that he has; a great moment no doubt for new viewers. Pearl Mackie continues to play Bill brilliantly; as I said in a previous interview, she really feels like someone who you would meet in real life. It makes a change to see a more inquisitive companion than the likes of Amy or Clara.

Unfortunately The Doctor's other companion, Matt Lucas's Nardole, is still being sidelined. In Thin Ice, he only appears in the final couple of scenes mixing coffee into cups of tea (which really should be a crime) and guarding the vault, and it continues to make Matt Lucas's involvement seem like a waste of time. Doctor Who Magazine put it perfectly: essentially his role in the series so far is little more than 'Matt Lucas cameo of the week'. It would be nice to be given a chance to form an opinion other than indifference towards the character of Nardole. Hopefully he'llfeature more in Knock Knock.

The show's SFX also suffers in this episode. The new series of Doctor Who generally has very good special effects, but post production company Milk's effort this week is poor. The SFX of the giant sea creature are consistently unbelievable (although the giant eye is impressive), as are the special effects of people falling through the ice. Doctor Who's low budget clearly shows here more than ever before; sadly it is a case of the show being over-ambitious with its visual effects.

Overall, Thin Ice is probably my favourite Sarah Dollard episode so far and definitely destined to be one of the most controversial episodes of the show. After The Pilot and Smile established the character of Bill Potts, Thin Ice is allowed to have a much stronger pace and has a conveys a nice Jules Verne-esque feel. It also makes a nice change for the threat to be earthly rather than alien, and Pearl Mackie is once again brilliant as Bill Potts. Unfortunately the episode suffers from yet another 'Matt Lucas cameo of the week' (as coined by Doctor Who Magazine) and surprisingly terrible special effects. These special effects are Invisible Enemy-levels of bad, and by far the worst of the new series.