Here’s a thing I vaguely recall about Pet Shop Boys’ third album, Introspective: somebody describing it as “the disco Sgt. Pepper's”. I don't remember who. But I do remember bridling at that assertion.

It’s so much better than that.

During what Neil Tennant famously called their “imperial phase”, and indeed for some time after it, Pet Shop Boys were by no small distance the best pop group in the world. They created something self-contained and perfect, as an idea, as an atmosphere, as a means of conveying a feeling and a sensibility. Each new hit was a miniature, immaculate facet of that larger whole, a cell containing its complete DNA – which double helix encompassed the scintillating melancholy of late disco, the thrill and danger of nocturnal city streets, the sway of the catwalk, the sugar rush of radios in teenage bedrooms, the feline shape of English queerness distinguishable beneath a translucent cloak of paper-thin ambiguity.

Tennant dated the end of that phase to the release of the first single drawn from Introspective, ‘Domino Dancing’, whose relative commercial failure – top-ten, in a period when that took serious sales; yet not a contender for the top spot itself, despite being the lead single for a forthcoming album – made him think, “That’s that, then – it’s all over.”

By “imperial phase”, he meant PSB’s ability to score No. 1 hits, rather than their artistic infallibility. That would continue for some time yet. Their fourth album, the subtle and more low-key Behaviour (1990), holds a deserved critical reputation that eluded it at the time. I recall being dazzled in 1993 by the fifth, Very, and struck by the profound undercurrent of loss in PSB’s first openly gay LP; arriving as it did shortly before HIV – which had killed so many gay and bi men and ravaged what was starting to call itself queer culture – ceased to be an automatic death sentence. “[Very] trails darkness in its sparkling wake,” I wrote at the time.

Five albums, then, to stand against any run by any act. And not that there is much to choose between them, but the third of them – the strangest, the most experimental, the least obviously “poppy” – is the greatest of the lot. It’s not just that Introspective is my personal favourite of theirs, although it’s that too; moreover it’s one of my very favourite albums by anybody. It spoke and speaks to me as if it had slipped a wire into my subconscious and drawn out all that is written there. It’s also that it accomplished a remarkable feat, at a moment when dance music was engulfing British youth culture in warm waves of hedonism: it is the only first-rate British club album to be made around that time by an act hailing from the pop side of the aisle rather than the dance side.

If at this point anyone wants to mention New Order’s Technique, or Original Soundtrack by S’Express, both of which emerged a year later, I don’t dispute the brilliance of either; only the former’s status as a club album rather than a club-literate one, while the latter was made by Mark Moore, who crossed over from dance to pop. (I’m using “pop” here in its genre sense, just to be clear, rather than in the broader sense which I usually prefer, embracing all forms of popular music from rock & roll onwards.)

Introspective is an oddity in so many ways, within PSB’s own catalogue, and within music as a whole. Some great albums are like that. They fall together around timing or impulse rather than execution as a entire piece. Each of those other four early PSB albums was a cohesive entity, conceived as such and drawing its character from it. Introspective was a grab-bag; only two of its six tracks were written for it. The other four were covers and/or recreations of previous PSB-originated numbers. It turned on its head the customary process whereby a band records an album, then releases and expands upon some of the songs as singles. The format which best suited Introspective was a set of three 45-rpm 12-inches, with one track to each side of vinyl (I lent mine to somebody, again I forget who, to DJ with, never got it back, and have regretted it ever since.) Because that’s really what it was: a set of club tracks, averaging over eight minutes apiece, which either reworked earlier material or would themselves be edited and remixed down for a more radio-palatable single release. Put the 7-inch versions together (and every track on it appeared on 7-inch, albeit one by another band), and you’d have a very nifty synthpop mini-LP. Put the 12-inch versions together – which is what PSB did – and you have an album of breathtaking depth and scope. (It’s fair to wonder if PSB were inspired by Soft Cell, and their Non Stop Ecstatic Dancing EP; while Introspective’s version of ‘Always On My Mind’ seems to borrow its structure from Soft Cell’s melding of their ‘Tainted Love’ cover and its b-side, ‘Where Did Our Love Go’, on that hit’s 12-inch version.)

In every instance on Introspective, the album version – that is, the club mix – is much the better one. On the two original tunes, ‘Domino Dancing’ and ‘Left To My Own Devices’, the differences are less obvious, but no less significant. ‘Domino Dancing’ is more snake-hipped and lissom, less clattering and insistent, than its single version. Might sticking with that sound have helped its sales, or was it simply too soon for Latin dance-pop to be a British chart topper? There may be an obvious example I’ve forgotten; or it may be that it was just too radical a departure for Pet Shop Boys’ audience, who had earlier that year pushed ‘Heart’ to No. 1, and the previous Christmas done the same with the band’s cover of ‘Always On My Mind’, formerly a melodramatic weepie best known in Elvis Presley’s countrypolitan version. Latin house did exist by then, but it wasn’t until the 1990s that it became a fully-fledged sub-genre, and the album version of ‘Domino Dancing’ feels like a bridge to it from early eighties Latin electro tunes.

As for ‘Left To My Own Devices’... well, I know people, people who love PSB as much as I do (which is to say, they cherish them and hear much of their own lives in them), who think it’s the pair’s best single. And I’m not saying they’re wrong. I’m just saying that when that glorious swirl of strings strikes up, and my eyes widen, and the hairs on my arms stand on end, and a sudden cocktail of endorphins and adrenalin kicks into my veins, and then instead of twirling upwards into the operatic soprano vocal of the album track it gives way to the beat of the 7-inch version... I’ll take it, but it’s just not the same. Because the version of ‘Left To My Own Devices’ that opens Introspective is the Platonic ideal of a Pet Shop Boys song (and, in its pristine precision, not far off that of a Trevor Horn production, too.) It has everything: the excitement, the melody, the poignancy, the joy, the glamour, the nuance, the literate wit. A note about that last one; happily, it’s been a long time since PSB were routinely sniffed at for being “too clever”, or “too ironic” – charges which could only ever have been levelled at them by people for whom any degree of either was always going to be too much. PSB have always been fearsomely clever, and on occasion have made splendid use of irony; but seldom has this rendered their music any less emotive. It’s a measure of just how ingenious they are that they could create song after song that resonated across generations, that struck the listener right in the heart, and do it without ever feeling the need to play dumb. Nothing illustrates it better than ‘Left To My Own Devices’; the contrast between Tennant’s typically underplayed vocal – here largely confined to a spoken near-monotone – and the surging, romantic orchestral arrangement packs a formidable sensory and emotional wallop.

For here is another extraordinary thing about Introspective. Dance music is by its very nature an extroverted format. It's about community, bonding, exhilaration, shared experience – and never more so than when house took off across the UK and ecstasy became its stimulant of choice. Yet Pet Shop Boys managed to make one of the greatest club albums of that golden period, and make it, as the title suggests, a distinctly introverted record; one dealing in seclusion (both sought and undesired), loneliness, and the darkest, most intimate aspects of relationship struggles. It's entirely in character for their own work, yet stuffed with floor-filling bangers. It's simultaneously a club classic, a pop classic and a classic of downbeat songwriting and interpretation. I can't think of another record from the time that achieved this. If you had ever been a solitary child “in a world of [your] own at the back of the garden”, finding your pleasures in your inner life and in visions of a very different future self; if you were never a joiner-in; if you were a creature of daydreams and uncertainty – here was your dance record. Pet Shop Boys were there for you, just as they had been before raving arrived.

Chris Lowe has told a story on himself and Tennant about a David Morales remix of their 1990 single ‘So Hard’, a “classic pumping house track” the hearing of which prompted Tennant to ask Lowe, “‘Why don't we make records like this?’ I said, ‘Neil, it is us.’ So that's how much we know about dance music.” He – intentionally, no doubt – does his band a disservice. Many other British acts (rock, pop and indie) latched on to dance music when the British rave scene took off, and for plenty it was plainly an awkward fit. Whereas in the pre-house days Tennant and Lowe were avid New York clubbers, and had sought out Hi-NRG pioneer Bobby Orlando (aka Bobby O) as their first producer. More to the point, they had made records like it, or not that far off. ‘Always On My Mind’ began as a audacious electro-pop revision of the Presley record, and was a huge hit, a Christmas No. 1; the version they rebuilt for Introspective was more radical yet. Combining it with the overt acid jam ‘In My House’, and using its soaring synth line to rapturous effect, they created a tune that was simultaneously equivocal and euphoric. Where Presley sang it as a man desperate to remedy his failings, Tennant sounds more like someone reading the list back to himself matter-of-factly, as if trying to work out where he stands. He ends on a note of tantalising opacity: “Maybe I didn’t love you...” It is impossible to quantify such things, of course, but I reckon the Introspective version to be almost exactly 527 times better than the one that topped the charts.

The transformation from serviceable synth-pop to tracks that are, if not outright house tunes, then certainly house-esque, is marvellously worked on ‘I Want A Dog’, which previously hovered on the fringes of novelty as the b-side of ‘Rent’, and here becomes a kind of poetic hymn to urban isolation; and on ‘I’m Not Scared’, where a song given to Eighth Wonder is reclaimed and made wholly the band’s own. This ‘I’m Not Scared’ is steeped in mystery, and in menace. “If I was you,” deadpans Tennant, “I wouldn’t treat me the way you do”; and one can only guess if this is a reproach or a threat.

But even if the band had been making any secret of their sources, the final track would have given them away. ‘It’s Alright’, by Sterling Void, co-produced by Marshall Jefferson, is a song they found on the third of FFRR’s superb The House Sound Of Chicago series, Acid Tracks. Again, there’s that note of ambivalence that makes Introspective such a work of chiaroscuro. Surveying all the trouble in the world on the original, Paris Brightledge affirms with powerful conviction that “it’s going to be alright!”, in the testifying gospel-soul style that was a feature of early vocal house tracks. “I hope it’s going to be alright,” hedges Tennant on PSB’s cover, by the end in a barely audible murmur as if over an iffy landline. What one knows, and what one hopes, are two very different things.

Pet Shop Boys’ masterpiece (and eventually their second-best-selling album globally) was kept off the very top of the UK album charts by its exact antithesis, Rattle And Hum, the most bloated and bombastic sackful of sub-mythic rockist piffle U2 ever recorded. It’s as if each band was most perfectly itself at the same precise instant. It’s an episode to file alongside Ultravox’s ‘Vienna’ being denied the No. 1 spot by Joe Dolce’s ‘Shaddap You Face’ and – to use a more apt Beatles analogy than the one mentioned earlier – the thwarting of ‘Strawberry Fields Forever’/‘Penny Lane’ by Engelbert Humperdinck’s ‘Release Me’.

I’d like to think this is what gave Pet Shop Boys the idea for their delicious non-album single of three years later, when they mischievously combined ‘Where the Streets Have No Name’ with ‘I Can't Take My Eyes Off You’ (more a cover of Boys Town Gang than Frankie Valli), and blew the U2 original out of the water, giving their version a far more dramatic widescreen sweep, then dunking it in a tub of camp bathos, which somehow only made it more uplifting. They weren’t just imperial, back then. They were imperious, too, in their own deft way. On Introspective, they gave their powers full range, and it is peerless.