The unique, two-decade story of the Pet Shop Boys is told here through a succession of their record sleeves and videos. It is, in turns, bizarre, ironic, silly, and always detached.

From the outset (in the late Eighties), Chris Lowe and Neil Tennant and their long-term collaborator, photographer and video-director Eric Watson (whose images dominate the show), were arch pop players.

Their art school backgrounds and Tennant's time at Smash Hits combined to create images incongruous with their infectious disco-pop.

Watson's photo-shoot for the cover of the debut single, West End Girls (1985), for instance, shows two young men posing in leathers and jeans in a dark King's Cross street and railway line.

Unashamedly gay, it avoids the kitsch-camp covers of Boy George or Freddie Mercury; it is straight documentary, though more Gay Times than NME.

Tennant and Lowe's typical lack of expression is perfectly suited to stills. Lowe tends to have his eyes shaded and, baseballcappedhovers behind the commanding, almost beatific Tennant. For It's a Sin (1987), Watson's stage-like cover setting, with its distressed décor, hints of mystery. Lowe averts his eyes, while Tennant stares like a romantic hero, evoking, he said, Whistler's Ennui.

Such references pepper the shots: Robert Mapplethorpe's geometric monochrome studio portrait of the pair, heavily inspired by Avedon, also plays with New York gay stereotypes, as leather boy meets besuited Wall Street trader. The band's fascination with Soviet imagery enters the story with Nightlife (1999), and Watson's tense portrait in another decaying room, with Tennant in a Cossack hat and Russian officer's coat.

Then came the triumphant camp of Go West, in which cartoon-like armies of Lycra-clad soldiers, Constructivist graphics and Village People choruses dwarf the lead characters, unembarrassed by their blue-and-yellow boiler suits and ridiculous colander helmets.

Only Pennie Smith's back-stage documentary shots from a 1997 tour offer insight into their characters, revealing Lowe smiling and Tennant laughing: a necessary companion to otherwise masked performances.

This modest exhibition of milestone images is a taster for the 2,000-plus photographs and video stills in the book Pet Shop Boys Catalogue (Philip Hoare and Chris Heath, Thames & Hudson, £29.95), a lavish reminder that they were always about more than music.

Until 28 February. (020 7312 2463, www.npg.org.uk).