Charlie Brooker doesn’t like it when folk accuse Black Mirror of taking itself too seriously. “Or when people say, ‘I want to do something like that, but with a sense of humour,’” said the creator of the sci-fi anthology series in a recent interview. “I’m like: ‘For f***’s sake, what’s wrong with you?’”So he won’t thank me for saying that the first half of "Rachel, Jack and Ashley Too", the second of this three-episode fifth season, is a little po-faced. It’s a fun idea – cast globally famous pop star Miley Cyrus as a globally famous pop star, Ashley O, whose entire consciousness is copied over into sellable robot dolls – but it unfolds, at first, at a strangely plodding pace, failing to make good use of its concept and its stars.Rachel (Angourie Rice) and Jack (Madison Davenport) are two oft-warring teenage sisters whose father is (for some reason?) a high-tech rodent controller, and whose mother is dead. Jack is sullen, has a septum piercing, listens to Pixies and rolls her eyes a lot. Rachel is shy and unpopular, spending her lunch periods sitting alone listening obsessively to Ashley O – “Hey, I’m a ho, I’m on a roll, riding so high, achieving my goals”. The song, to be fair, is pretty darn catchy.When Rachel sees an advert for an Ashley O doll, “Ashley Too, based on Ashley O’s actual personality”, she begs her father to buy her one for her 15th birthday. He obliges, much to her sister’s annoyance, and Rachel is buoyed by the earnest platitudes her new friend doles out, seemingly directly to her. “You can do anything if you put your mind to it.” “You’re a special person.” Rachel is quickly, worryingly, enamoured.The real Ashley, though, is growing sick of the bright, shiny front she’s been trained to put on by her domineering aunt Catherine (Susan Pourfar). When she starts to rail against the narrow confines of her pop persona – something the real Cyrus has been doing since her tween Disney days – things go very wrong for her. Her fate goes from bad to worse, and the episode – or “film”, as co-producer Annabel Jones calls them – starts to feel very bleak indeed.Until, quite suddenly, the tone changes. Jack and Rachel accidentally remove the “limiter” that was only allowing Ashley Too to operate at 4 per cent of its capacity. Suddenly, the “real” Ashley is barking out expletives from the tinny speakers of her robot counterpart. “They copied my entire f***ing mind into these things,” she tells the astounded sisters. What follows becomes less a meditation on the ethics of posthumous profiting and the commodification of vulnerable young stars than a fun, high-concept heist film. It is an enormous relief – not because those meditations are not worth considering, but because the energy up until now, aside from an explosive, well-wrought argument between Ashley and her aunt, has been strangely subdued.Cyrus, who according to Brooker, took the role to “p**s people off”, is excellent, most evidently when at the two extremes of her performance – desperate and broken, or uninhibited and defiant. Rice and Davenport improve hugely once their characters evolve beyond thinly sketched cliches.As is quite often the case in Black Mirror, there is a slight over-abundance of ideas in "Rachel, Jack and Ashley Too". Plot points are picked up and dropped; narrative arcs lead to nowhere; unnecessary characters abound. But it is certainly worth sticking with – even if just for the three, Fleabag-esque final words. I won’t spoil them for you.