With Doctor Whos popularity at an unprecedented high, replacing the lead was even more of a risk now than it usually is. This makes the task Peter Capaldis debut episode, which premiered in Cardiff on Thursday as the first stop in the shows world tour, all the more difficult. It has to convince the viewer that this new man is still the Doctor, whilst distinguishing him enough from the last incarnation to stop it seeming like a cut-and-paste recast. The last time we welcomed the next Doctor, it was a breathless race to establish the new guy, introduce a new companion and her boyfriend, briefly tour a rebuilt TARDIS console room, destroy then bring in a redesigned sonic screwdriver and establish the story arc for the series. The title Deep Breath is fitting then as, with a continuity in companion and production team, the episode uses its extra breathing room to focus on the new Doctor and exploring the altered dynamic between him and Clara. Moffat has been keen to emphasise that casting an older actor as the Doctor does not mean a shuffling, low-energy performance for the Twelfth Doctor. Certainly, our first few scenes with the new Time Lord showcase all the madcap energy weve come to expect from recent incarnations, but its the calmer moments where Capaldi really shines as the Doctor. Every line is delivered with tremendous weight and authority. Even when the Doctor is being daft or eccentric (which is often), Capaldis performance shows a dignity quite unlike the raw physicality and childlike glee of his predecessor. The extra millennia or so that the Doctor clocked up on Trenzalore seems to become manifest in this incarnation at times, with signs of a weary melancholy and even nostalgia in some of the quieter scenes. Its difficult to get a handle on the Twelfth Doctor from only one episode, but this story gives the impression that the new incarnation will be defined less by quirks than his predecessor. Theres no fish fingers and custard. No bow ties. No catchphrases (yet). He is a forthright, no-nonsense Doctor. Clever, commanding and at times a little cold, the Twelfth Doctor is unsentimental and pragmatic to a fault. Moffat toys with our uncertainty about the new Doctor to create some truly jaw-dropping moments and a conclusion to the main plot that throws one of the most integral part of the Doctors character into question. By the time the credits roll, theres no doubt that this is not the same man but, through the worst-kept surprise cameo in Who history, you're able to see him as the Doctor. The episode clocks in at an exhausting 75 minutes, making it the longest introductory episode to a new Doctor besides the McGann TV movie. Unfortunately, the script seems unable to justify the length and there are signs of padding when the pace drops to a laboured crawl just after the first act. Though this does allow for some much-needed development of Vastra, Jenny and Strax - as well as allowing the Doctor to mark out some contrasts with his predecessor - it felt more like a bizarre diversion into alien Downton Abbey than part of the episode. It has great character moments and gets everyone in place for a protracted set piece that makes up almost the entire second half of the story, but it doesn't fit with the rest of the episode. Series eight returns to the full thirteen-episode format that Russell T Davies used so effectively. Questions are raised that will likely build over the course of the series and culminate in the finale, though a major plot development from the 50th anniversary - which most people had assumed would be the driving factor of this series - isn't referenced and it remains to be seen how or if it'll play into the overall story arc. Nevertheless, theres more than enough intrigue to fill the coming episodes. One particular question, raised simply by Capaldis casting, is mentioned but thankfully not hand-waved away as flippantly as it could have been. As a story, Deep Breath starts as an awkwardly paced jumble of scenes and plot points that gradually fall away to leave a tense and exciting climax. As a debut for the Twelfth Doctor, its laced with meticulously-written moments that give us a sense of who this new incarnation is and how he will interact with the other characters around him. If you've resisted the urge to peek at leaked scripts or suffer the rough cut thats been knocking around torrent sites for the last fortnight, you wont be disappointed by the wait.