'That's what Vince has done, isn't it?' said Purbrick. 'He's been forced to swap night for day, going out when everyone else has gone to bed.'

Maggie turned the page back and pointed to a reproduction of Evening. 'Diana and her cuckold Actaeon. We know this is who the painting is meant to represent because the characters are painted on the fan held by the central character. The spirit of Diana hangs over the picture, but in a sort of perverse parody. Actaeon was changed into a stag for looking upon Artemis while she was bathing, but here he's been turned into a cuckold. He's standing in front of a bull.'

'What does that mean?' asked Purbrick.

'It looks like he has horns. Putting horns on a man is the traditional manner of showing that his wife is unfaithful, Stanley, surely you know that.'

'I don't see how it helps us,' Purbrick complained.

'According to these notes it's supposed to be five in the afternoon because there's a milkmaid milking a cow in the background, and we're at Sadler's Wells. Those are the hills of Highgate in the distance.'

'Not much help. Turn to the next one.'

'Noon,' said Maggie, holding up the third reproduction. 'Ruled by Apollo, associated with the sun, and Venus, whose love is inverted here to be symbolised by a vulgar marriage and various earthy lusts, lots of sloppy feasting and debauchery.'

'I like this one,' said Purbrick. 'It's a bit saucy. Where the good woman is silent. Who is the good woman, then?'

'She's on the sign outside the inn.'

'But she's got no head.'

'That's what makes her good; she can't nag.'

'Hmm. What about the last picture?'

'Morning. The church dial says five to eight. An old maid crossing Covent Garden Square. She's dressed in yellow, the colour of the dawn sun, like the goddess Aurora -'

'Like the fire that Prometheus himself carries,' added Purbrick.

'That's right. She's on her way to church, but this is another inversion, another cruel paradox. She's peppered with black spots, venereal marks, and dressed less for religious service than personal vanity, hence less pious than she appears. She looks more like she's been out all night and is going to church to repent her carnal sins.'

'Well, the symbolism of the paintings is obvious, then, isn't it?' said Purbrick. 'He's commenting on Vincent.'

'How do you mean?' asked Maggie.

'The four goddesses, they all represent some aspect of what Sebastian perceives to be Vincent's world tonight. Persephone runs into the night and into the underworld, Diana the huntress takes on a challenge, Venus the lover turns the love into sin, and Aurora is the dawn-bringer, carrier of fire. Vincent befriended Sebastian, but the friendship soured under false pretences, that's Venus -'

'It's a bit different to love, though, isn't it?'

'Not in the classical sense, not at all. Friendship between males is seen in terms quite as strong as love, and governed by exactly the same rules of loyalty and duty. Next, Vincent ran into the night at Sebastian's instruction, he accepted the challenge and hunted through the fields for him -'

'Fields, Stanley?' Maggie raised an eyebrow.

'Indeed, for what will you find beneath the concrete of the city? Fields, Margaret, fields and the bones of Londoners. But according to these pictures, there's one thing Vincent hasn't done yet.'

'He hasn't brought in the dawn.'

'Right. That's where he has to go, where the character in the fourth painting is going, to Covent Garden, to the portico of St Paul's Church, featured in the picture. At least, that's what we're meant to think. But it's another trick, you see.'

'You mean because the painting is actually back to front. Hogarth always drew in a mirror.'

'No, no, it's to do with the church. I used to attend there when I was younger, so I know this is true,' said Purbrick proudly. 'Inigo Jones outraged the clerics when he completed St Paul's in 1633, because he placed the altar at the west instead of the east end. It was something quite unheard of. He wanted his grand portico to face east into the new piazza, but in the face of so much ecclesiastical protest he resited the altar without changing the church's overall design. By doing so, he turned the church back-to-front, transforming the grand portico into a fake doorway, reducing it to a mere stage, where street entertainers now perform to the crowds.'

'Good heavens.'

'He created a paradox, a backward church that almost mocks itself with jugglers and fire-eaters instead of holy men.'

'Stanley, there's life left in you yet,' muttered Maggie, amazed.

If he reached up he could feel the curved brick ceiling at his fingertips, but there was no break in the water above him. The tunnel was completely filled. Another jutting stone ledge scraped past Louie, and he twisted his body, mindful of the metal rod still protruding from his thigh. He was running out of breath. The trick was not to panic. He allowed his body to go limp and be carried by the buffeting current.

The tunnel swept sharply to the right and up slightly, so that he could feel the slime-covered base of the sewer rapidly passing beneath his boots. Moments later he burst from the freezing water into air, propelled to a hard landing on the far side of a large iron grid, down through which the overflow from the tunnel was rushing. A faint grey light showed from above, allowing him to make out the hexagonal shape of the broad Victorian air-shaft. He reached down to his thigh and found the steel shaft of the arrow gone, wrenched free in the vortex.

Things got better after that. Pam had been pulled loose in his fall and now appeared behind him, although he was too late to break her fall as she burst from the tunnel and landed on the drainage grid. It took all his strength to drag her free of the torrent before she could swallow any more effluent. Water also spattered his face from above. Rain was falling through the large iron pores of a rectangular drain lid. Pam was not conscious, but was at least breathing hard. He rested his head against the green-slimed wall and allowed the falling raindrops to wash the filth from his face. Then he pulled Pam against the wall at his back and began looking for the ladder that he knew would take them to the surface and safety.

It was only when he opened his jacket to squeeze some of the water out that he realised what had happened. The disk he had fought to save, Vince's final copy of the manuscript, had been sucked from his clothes and washed away, the fragile magnetic square lost in the surf of detritus thundering through its channel beneath the city.

'Interesting,' said Bryant, 'this bit here about the inquest.' They had tapped into the only report that had so far made it onto the Internet, a web-site dedicated to celebrity criminal cases. He tapped the scrolling screen with the end of his biro. Jane shifted closer on the piano stool they were awkwardly sharing. The coroner's report on Melanie Daniels suggested that there was no sign of panic in the victim. Drowners usually start to hyperventilate when they inhale water into their lungs. No cadaveric spasms, few diatoms. It's not symptomatic of drowning. I'm surprised the verdict was misadventure.'

'I'm not sure what you're looking for,' said Jane.

'I'm not sure myself, but this is all very suggestive. Daniels was relaxed when she went into the water, if not unconscious. How could that be?'

'She must have been pretty out of it.'

'Oh come on, the party was late in the year, the lake would have been freezing cold. If she was so drunk and stoned that she couldn't even feel the icy water pouring into her mouth, how did she even manage to get as far as the end of a narrow jetty in semi-darkness?'