Imagine The A-Team but instead of a band of wrongfully convicted US army commandos who become soldiers of fortune, it’s a group of dissatisfied baggage searchers and VAT investigators who have taken their ties off. Are you sold? Good! Because Legends is a six-part thriller by Neil Forsyth based on the true story of a group of ordinary men and women recruited from the rank and file of Her Majesty’s Customs in the early 90s, given three weeks’ training and sent undercover to infiltrate and bring down two massive drug cartels that were filling Britain’s streets with heroin and really pissing Mrs Thatcher – head of the party of law and order, don’t you know – off.

Steve Coogan – possibly in need of a spot of emotional relief after a career spent playing losers or Jimmy Savile-shaped villains – stars as former undercover police officer Don Clarke. He puts the team together for the home secretary (Alex Jennings – this is statutory) and HMC’s director of investigations Angus Blake (Douglas Hodge) despite neither of them seemingly offering any money or support for the project.

After a fun, if slightly silly, trot through the recruitment process – anyone who asks about overtime or the finer legal details of what they’re being asked to do is summarily ejected and only those who put in the extra hours at lockpicking class can expect to make the grade – Don has found his people and we can get into the meat of the thing.

Guy (played by Tom Burke, a fine actor who has chosen a Mogadon monotone and dragged-through-molasses energy that becomes increasingly irreconcilable with the character of Guy, as he rises through the gangster ranks by his wits and fists) is recognised by Don as a “lone wolf” operator. Don sends his protege to London with dire warnings not to let his “legend” – the fake identity issued to all recruits, which they have to believe in as wholeheartedly as possible to protect themselves – and the darkness of his new life overtake him. Guy is to pose as an importer of drugs and inveigle his way into the vast operation run by Turkish overlords who are hoping to expand beyond the relatively nugatory amounts and profits that can be brought in via human mules. Off he trots, leaving behind a daughter and extremely understanding wife, Sophie (Charlotte Ritchie), who did a brief investigatory stint herself back in the day and knows about the allure and risks.

Two other recruits – hardbitten, hotheaded Essex native Kate (Hayley Squires), who has become sick of tracking down perverts and their illegal stashes of pornography, and the more thoughtful, tentative Bailey (Aml Ameen), equally tired of life hunting down VAT evaders – are sent to Liverpool to see what they can find out about the drug gang controlling the streets there.

Last but not least is Erin (Jasmine Blackborow), backroom data hound extraordinaire, sniffing out evidence trails and helping the legends stay half a step ahead of the increasingly threatening people around them. Corrupt cops, last-minute patches to stories, tiny slip-ups, missing door codes and gangland power struggles keep viewer nerves jangling nicely.

It is a great story – plucky underdogs risking life and limb for noble ideals (a drug-free country! The protection of impoverished, vulnerable and innocent people!) while baddies wait to pounce on mistakes – and the establishment are braced to grab any credit for themselves. You can see why Forsyth (best known for The Gold which dramatised the Brink’s-Mat robbery) and its aftermath, which touches on some similar state-of-the-nation themes) was drawn to it. He mostly, if sometimes very, very narrowly avoids falling into the ever yawning trap that a story about customs officers becoming the A-Team inevitably faces, which is the potential for bathos, if not outright risibility. “You think a few customs officers can take on the biggest drug gang in Britain?” (as the Home Sec says to Blake) is a line that could easily come from a sitcom or a comedy sketch. “From tomorrow you’ll disappear. Tonight, say your goodbyes” should be in a high-octane action thriller starring at least a B-list American as something cool. Into the gulf between them, credulity risks falling – especially given the presence of Coogan, good though he is. At the back of the viewer’s mind is always the suspicion that he is about to unleash his comic chops and change everything.

The energy spent keeping things serious prevents the series catching fire. But it remains a brilliant story, here well told.

• Legends is on Netflix