This has been one of the best Doctor Who episodes of the season, but feels too little, too late for Ruby Sunday (Picture: BBC Studios/Bad Wolf/Lara Cornell) Spoilers ahead for Doctor Who episode four, Lucky Day. The latest episode of the hit BBC sci-fi series returns to present-day Earth, where we check back in with former companion Ruby Sunday (Millie Gibson), who is settling into normal life after discovering her biological mother at the end of the last season. The episode, unexpectedly, is the best of the latest Doctor Who series so far… but it exposes a big problem that I have with the show’s current era. Now she works with Unit to help defend the planet from alien threats as she tries to cope with the trauma of being on the brink of life and death more than once during her time on the Tardis. Enter Conrad (Jonah Hauer King), a plucky young podcaster who has been fascinated with the Doctor and extraterrestrial life since encountering the Doctor (Ncuti Gatwa) and Belinda Chandra (Varada Sethu) as a child (timey-wimey). After managing to convince Ruby to chat with him about life as a companion and the threats Unit is protecting the world from, the two spark up a sweet romance filled with plenty of chemistry. The show does such a good job convincing you that the twist will be Conrad dying after failing to protect himself from the alien hunting him… so much so that the delicious plot twist that he is actually the bad guy is such juicy storytelling that it keeps you hooked for the rest of the episode. Millie Gibson and Jonah Hauer King have wonderful chemistry in the new episode (Picture: BBC Studios/Bad Wolf/James Pardon) That makes the reveal all the better (Picture: BBC Studios/Bad Wolf/Lara Cornell) It’s classic Doctor Who – character whiplash, fight against evil from the unexpected places and layered social commentary that is capped off with an epic speech from the Doctor himself. Throughout, Millie’s acting – reminiscent of her powerhouse performance in the episode 73 Yards – makes this one of my favourite Ruby Sunday episodes yet. This slightly hardened version of Ruby beautifully explores how genuinely traumatic it would be to be flung in front of alien threat after alien threat at 19. Her devastation – combined with gritty determination – to push against Conrad when he shows his true colours is excellently pulled off. Seriously, I was mesmerised. Ncuti Gatwa delivers a powerful monologue to cap off a powerful episode (Picture: BBC Studios/Bad Wolf/Lara Cornell) It’s why I’m so sad Russell T Davies has waited this long to elevate Ruby’s character to these heights. Although she had a fun first season, the shortened number of episodes and unfortunate lack of character development meant she previously felt 2D at times. It almost feels too little, too late to have such a high-quality episode near the end of the time on the show. We’ve been given a taste of how great Ruby could be, with no promise of ever fulfilling her full potential on screen. Perhaps if she had been the full-time companion for a second season, there could have been a chance to explore Ruby and her relationship with the Doctor’s life further. I only wish we could have seen Millie Gibson’s full potential as Ruby Sunday before now – and had the chance to see more (Picture: BBC Studios/Disney/Bad Wolf/Lara Cornell) Instead, we’ll have to make do with the taste we have been given so far. Meanwhile, although this is a Doctor-lite episode (again, an unfortunate choice when we already have such a reduced amount of time with Ncuti), his end-of-episode monologue is impactfully delivered. Especially face to face with Jonah, who – far from his days as a Disney prince in The Little Mermaid – brilliantly captures Conrad’s villainous undertones. He’s not a maniacal alien deadset on wiping out the human race. His evil is in the dangerous manifestation of his radicalisation – unable to see sense beyond his own echo chamber. All in all, this episode blew me away with its brilliance, but I couldn’t help but feel bittersweet for all the epic episodes we could have seen with Ruby if we had the time and chance. But if Doctor Who maintains this quality for the finale, the show could save itself. Doctor Who will return next Saturday. Episodes are available to watch on BBC iPlayer. Got a story? If you’ve got a celebrity story, video or pictures get in touch with the Metro.co.uk entertainment team by emailing us celebtips@metro.co.uk, calling 020 3615 2145 or by visiting our Submit Stuff page – we’d love to hear from you.