Travel: Japan - Inside the giant electron

British Airways has announced the best-ever deal to Tokyo and back - pounds 300 return. Simon Calder recommends the ultimate urban experience

"Never trust a city you can walk across in under an hour," a resident of Rio once warned me. She would place complete metropolitan faith in Tokyo; by some measures, the Japanese capital extends for 300 miles, to embrace Osaka in the world's greatest megalopolis. Even the most hardened city-dweller could find that scary - until you find yourself swept along in the swell of humanity surging through the city and discover that, up close, Tokyo looks after you like no other city.

The challenge is to decipher it. After several visits, I realised that the code is absurdly simple: a circular railway called Yamanote. Unlike London's Circle Line, the Yamanote is elevated. Take a couple of circuits to assess the city, and gasp at the staggering scale of Tokyo.

The security derives from the ease with which the city breaks down into small, manageable chunks. Around each Yamanote Line station clusters a community to be explored with safety. As with the alphabet, each of the 26 components is pleasingly distinct.

And if you begin alphabetically at Akihabara, you'll start small, too. The first station north of the main Tokyo station gives access to a jumble of electronics shops furiously selling devices at prices that would put Dixon's out of business (with the pound strong against the yen, the temptation to exceed your pounds 145 duty-free allowance will be a problem). A bit beyond the excessively bright lights, you can pace down intriguing arcades where commerce simmers more sedately, and where the neon is softened by an elegant crimson arc indicating a Buddhist shrine.

Board another anticlockwise train to Ikebukuro, Tokyo's closest trendy approximation to Camden (though the north London district has fewer tall buildings and, in my experience, not a single "capsule hotel"). If you were looking for some sort of edge in the Japanese capital, you might find it here. Wander around Rio wearing a bemused expression, and you are almost bound to be robbed; try the same in Tokyo, and you are certain to be helped. Should you know exactly where you are going, smiling young people on street corners will hand you small packets of paper tissues. This is not an ancient tradition of hospitality but a marketing technique; like most of the available surfaces in Tokyo, the wrappers carry advertising.

That the Japanese capital is like nowhere else on earth becomes confirmed if you stay on the Yamanote to Shinjuku, and track down the Number One Building of the Metropolitan Government Office - which has a free viewing- platform on the 45th floor.

In wilful defiance of the tectonic plates that creak beneath Tokyo's surface, Shinjuku is an exercise in elevation. Skyscrapers crowd the foreground, causing eddies in the sea of humanity that washes around their bases. Yet even at ground level you do not feel like a humble electron on a giant circuit board. You probably feel like a walk.

How about a stroll across Tokyo, at least the central core as defined by the Yamanote Line? The journey could take most of the afternoon and evening. You may pause to spectate at Octopus Army in Harajuku, a shop where wayward Japanese youths express their uniform desire for individuality. Then wander through the serene cemetery of Aoyama.

As the sun slides through the heat haze towards where the horizon once lay, you can stumble down into Roppongi, Tokyo's stab at Soho sleaze. The first two ingredients of the sex/drugs/ rock'n'roll recipe for indulgence appear to have eluded the Japanese, giving Roppongi a wholesomeness that the note-perfect Beatles tribute bands can only reinforce. In Rio, the later it gets, the more the temperature and tension rise. Nightfall in the Japanese capital calms the city and eases the heat. A breeze drifts in from Tokyo Bay, the flickering fades and the skyline settles into a fixed constellation of electric light. May it never be switched off.