Stare into Charlie Brooker’s Black Mirror for long enough and you might not like what is staring back at you.

For yes, his vision of the future – mapped out in series one with ‘The National Anthem’ (voyeurism, pig-bumming), ’15 Million Merits’ (a reality TV dystopia) and ‘The Entire History Of You’ (a relationship destroyed by technology) – is shocking and dark and perhaps even a bit ridiculous, but it is also grounded in the uneasy relevance of reality. After all: it’s a stark, reluctant truth that if the drug of technology has us hooked, then the technological age is moving too fast for us to truly know the side-effects. And that is the reason why, as the eerie silence of end credits drifts over the screen, ‘Be Right Back’ lingers hauntingly in the mind.

Just as the first series followed the thematic format of a story that rooted itself in politics, a dystopian wider-concept episode and a huge science fiction idea focused down to one relationship, so does series two – although in a different order. Here, we open with the template of the latter: a couple who, after sudden tragedy, are one of millions of stories in a near-future where advances in software and synthetic flesh has given way to a disturbing prospect.

The couple in question are Haley Atwell’s freelance artist, Martha, and Ash (Domhnall Gleeson), a cheeky, loving boyfriend whose penchant for social media means, to Martha’s annoyance, being easily distracted by his mobile phone. Still, their relationship is one defined by the all-consuming, burning madness of love; hence why they’re moving to a remote cottage for a blissful new life together. Everything is perfect. And that’s when death – especially the sudden kind – is all the more brutal.

Alone and ravaged by grief, Martha is signed up by a friend to a new online service where Ash’s internet activity – “he was a heavy user” – can be used to replicate his personality in an instant messenger program. This would allow her to talk and have it respond in the way he would. At first, she’s disgusted, but then something happens. And it’s then, desperate and with no one else to turn to, that she talks to the only person who can help. It’s a slippery slope.

Even in its earliest IM guise, the technology presents a darkly personal scenario – asking its audience, most of which will have experienced the pain, longing and rationale-blinding fog of grief, if they would do the same. Some, through visiting psychics, already do. From there, however, it gets creepier. The IM service upgrades to phone calls – his voice harvested from videos – and then, in its ultimate form, to a sort of synthetic version of Ash himself. It’s just like having him back again. But with his personality pulled from his web activity, is it really him? Are any of us the same in real life as we are online?

That, overall, is the major theme of ‘Be Right Back’. It’s an idea that’s Philip K Dick in its intelligence and ambition, but, in its execution, is not rammed down your throat in the pursuit of Making A Point. Instead, it’s written, directed (Owen Harris of Misfits and Skins) and performed with an intimate sense of slow-burning subtlety. The near-future they inhabit is dotted with little touches – a thumbprint identity scanner of parcel deliveries, for instance – that makes the world believable as, potentially, our own; the news in the background talks of wider ethical issues of synthetic flesh – showing that this couple in front of us are just one grain of sand on an ominous beach; and then there’s Synthetic Ash himself.

His unravelling is gradual and clever, with the distinctions between Real Ash and Synthetic Ash not being huge winks to camera, but the tiny changes in his personality that seem insignificant to the outsider (at one point, he tweets that the Bee Gees are “cheesy” whilst singing their guilty pleasures shamelessly with Martha), but not to the person who really knows you. The one who you’ve shared the deepest, darkest parts of yourself with – the parts you wouldn’t let anyone else see. When the Bee Gees play on the radio later on, Synthetic Ash responds with a sneery sense of disgust; as befitting of the ‘cool’, stage-managed projection of himself that he perpetuated online. It’s the little things that matter.

As a result of such subtly, there are no gimmicks or sharp turns or idealism here. The situation is what it is – and that, in the end, is what makes it all the more horrifying. Make no mistake about it: this is television that leaves you thinking. This is science fiction at its sharpest, subversive best.