We need a new phrase – the opposite of the overused “jump the shark” – to describe the moment when a TV series all falls into place; when we realise that we now not only know and understand these characters but are reaching the moment that defines their lives for us. In America, “growing the beard”, alluding to an upturn in quality for Star Trek: The Next Generation which coincided with the appearance of Commander Riker’s face fuzz, has gained some traction. But just getting really good is not quite what’s going on with Cucumber.

The latest episode, which broadcast on Thursday night, with its shocking, hammer-blow denouement, has proved a game-changer. Enough so we might adopt “doing a Cucumber” as the best way to describe the moment a show you think you’ve got a handle on, suddenly turns into something altogether different, more provocative, and emotionally overwhelming. Episode six of Cucumber was the most sublime yet devastating hour of TV I’ve watched in a long, long time.

Irritatingly, the series hasn’t exactly been a massive hit for Channel 4 (although it’s been getting around the same figures as the stupendously funny Sharon Horgan/Rob Delaney comedy Catastrophe, which also underlines how ratings are no reflection of quality). The problem is it’s as tough as ever to sell a drama which isn’t about doctors or detectives or people swanning around in beautiful period costumes amidst lush backdrops. Even Channel 4’s astonishing conspiracy thriller Utopia failed to catch on with viewers.

But if there was some way for the broadcaster to alert anyone who cares about good dramas to the fact that this week’s episode of Cucumber is a stunning piece of work, they really should. Perhaps a Breaking News alert in the middle of Hollyoaks, or something.

Despite some inevitable complaints from those unhappy about how a series ostensibly about a bunch of gay men has “represented” gayness (as if its Russell T Davies’s fault that there have been hardly any TV dramas with an LGBT focus since his own Queer As Folk 16 years ago), for me Cucumber has been a total delight from the start. It’s been reminiscent of This Life in presenting a group of multi-layered, not traditionally likeable characters whose sexual exploits are depicted as honestly and unflinchingly as the moments of violence which define crime dramas. I’ve been enjoying its giddily freewheeling, often farcical tone; its unapologetically blunt account of its characters’ various issues with sex (fear, shame, avoidance, addiction); and its spikily uncompromising lead character played with total conviction by Vincent Franklin.

But this sixth episode, which both formally and tonally takes a new direction, stopped me in my tracks, casting everything that’s happened so far in a new light. And, having also seen the final two episodes of the drama, I can also promise that the fallout is used by Davies to gnaw away at the very core of his characters in heart-rending yet spectacularly powerful style. It’s also further evidence that long-form TV drama can take the kind of thrilling narrative and tonal risks which most films understandably avoid.

So put your prejudices to one side, cast away all doubts and make sure you catch the final three episodes of Cucumber. They’re not only the best thing Russell T Davies has ever written, they’re as bold and brilliant as TV drama gets.

• Cucumber continues on Channel 4, Thursdays, 9pm. Catch-up on 4oD.