Diana Quick makes a return to the big screen – after a six-year break – with a first-rate performance in a second-rate (perhaps third-rate) film. Forever Young is a ploddingly drab British sci-fi drama that really doesn’t have what it takes to keep up with Quick; she stars as a 70-year-old writer taking an unlicensed drug to reverse the ageing process. It’s impossible to take your eyes off her: intellectually engaged and emotionally transparent. It’s a pity then that the script is unserious and uninteresting, with nothing to say either about the existential leap in the dark of eternal youth or the ethics of the anti-ageing industry.

Quick is Robyn, a bestselling author who’s been happily married to Oscar (Bernard Hill) for donkey’s years. Robyn’s big regret is her decision not to have kids. Then, one day, she gets a shock opening the door to an old flame, Jim, who miraculously looks 40 younger. Jim is a scientist who has invented an anti-ageing drug; now he wants Robyn to take it. She agrees, but husband Oscar refuses. At which point Quick exits, replaced by Amy Tyger as her younger self, and (no offence to Tyger) the movie gets very slack.

You have to put up with implausibilities and plot-holes by the shovel load. Robyn takes the age-reversing drug because she wants to have a baby – but doesn’t appear to ask a single question about the possible side effects. There’s also a painfully unconvincing subplot involving Jim’s daughter, an aggro drug addict and sex worker. In the end, this is a shallow drama passing itself off as saying something meaningful. It left me empty and the only question I was asking by the end is, can someone please write a stunning lead role worthy of Diana Quick?

• Forever Young is released on 26 January in UK cinemas.