From mob murders to snakes in letterboxes, this journey through the underbellies of London and Tokyo should be a smash. How have the BBC and Netflix managed to make it such a bore?

Younger siblings, eh? They are either the attention-sucking baby of the family, leaving you out in the cold, or a thorn in your side causing trouble from cradle to grave. Detective Kenzo Mori feels your pain. When a Yakuza boss is killed on Mori’s beat, threatening to destroy the fragile truce that has been maintained for the last few years, it appears that the killing may have been undertaken in revenge for a murder in London that Mori’s younger brother, Yuto, carried out.

Yuto was believed dead, but nobody ever saw the body and there has been a rumoured sighting of him. Mori’s boss – a man possibly ensnared in the Yakuza web himself – requires his underling to go to London and pretend to be a student on a detective refresher course while he does some unofficial digging. The theory goes that Yuto is more likely to reveal himself to his brother than anyone else.

So begins Giri/Haji (Duty/Shame), an intercontinental thriller and co-production between the BBC and Netflix. The latter – which will air the series globally next year – provided the deep coffers needed to split the setting between Tokyo and London. Mori is played by Takehiro Hira, who gives the detective a bone-deep weariness that makes increasing sense as we see, via a series of flashbacks involving Yuto, that it is not just the burden of ageing parents and a rebellious teenage daughter weighing him down. (Yosuke Kubozuka lends Yuto a needy, nervous energy that justifies what might otherwise seem unconscionably extreme service from his brother.)

In London, Mori puts out feelers among the Japanese community in his search for leads. A half-Japanese young man, Rodney, proves a useful conduit, but Mori discovers he is also a rent boy with a complex and dangerous life, in which he soon becomes embroiled. In a scene-stealing turn from Will Sharpe, Rodney is full of brio and wit (“I’ll have to ring you back,” he sighs theatrically into his phone when an emotional ex-boyfriend turns up. “I’m in the middle of a Greek play”). But he also has a vulnerability that makes us care instantly about his fate; possibly too much, given his ability to get into perilous situations soon and, one fears, often.

At a club Rodney introduces him to, Mori hears rumours of an assassin who fled Tokyo and is making his mark in London. He sets out to pursue the lead below the radar. Above the radar, he soon deduces, he is being watched and, where possible, thwarted. The plot thickens.

Meanwhile, his refresher course is being run by Detective Sarah Weitzmann (Kelly Macdonald, using her patented form of stoic anguish to full effect), a lost soul in her own city; ostracised at work and harassed at home – albeit inventively, with snakes pushed through her letterbox by ill-wishers. So far, she and Mori have done little but exchange awkward pleasantries. But whatever continent the protagonists come from, we have been here before and international relations will surely take place before series end.

Herein lies the problem with Giri/Haji. Apart from the novelty of seeing Japan’s capital unfetishised – this is a Tokyo where people live, work and manage the daily grind, not a neon-soaked fun palace or futuristic hellscape – and the odd animated interlude (created by the company behind Hey Duggee, fact fans), nothing here feels new or revelatory.

Lonely, troubled detectives torn between loyalty to a ne’er-do-well relative and their professional duty are a staple of police series – and they are often in unsuspecting conflict with corrupt powers above them, too. For the fish out of water to be Japanese rather than American or European provides some nice cultural cosmetic detailing – like Mori bowing to the taxi driver who drops him off after his first black cab ride, and the man speeding off before he has even straightened up – but the bones are the same.

So far, at least. Not all the episodes were released for review, though they will all be available on iPlayer after the first has aired. I am hoping that the actor playing Roy, the London detective sent to Tokyo in Mori’s place, gets something more to do. Being cast simply so one of your new colleagues can deliver the line “He looks like a fucking sex tourist” seems harsh.