'They have to be something to do with animals,' said Stanley Purbrick, pulling at a loose thread in his cardigan. 'Although I've never heard of anything called a Murder or a Bevy. There are animals in Lombard Street, hanging from the signs, gold locusts and frogs and things.'

Maggie was nearly asleep. Her face was sliding down her arm. Jane set another mug of coffee before her on the table. She took a sip and forced herself to perk up. She stared blearily at the list. 'They're collective nouns,' she exclaimed, surprising even herself. 'You know, like an "unkindness of ravens". Where's the thesaurus?'

They eventually found the collective nouns catalogued not under 'Animals' but 'Assembly', an entry which in itself required a certain amount of lateral thinking to locate. In another minute or two, however, Maggie had translated the list back into the animal kingdom:

Larks, pheasants

Woodpeckers

Penguins

Crows

Buffalo

Seals

Herons

Jellyfish

They called Vince and got through just as he was alighting from the taxi a hundred yards or so from the deserted main gate of the zoo.

'You're going to have to climb over the railings and avoid the night watchmen for this one,' said Masters. 'I don't think you're meant to head for the monkey house. The chimpanzee poem was just to get you to the zoo. The list contains the pointers. Try the buffaloes. We think they're on the far side of the gardens.'

'Wait a minute, hold on here.' Purbrick raised his palm. 'Why send him after the buffalo? How do you know it's not, say, the jellyfish?'

'The buffalo is the only four-legged creature on the list,' said Maggie, rolling her eyes in exaggerated impatience. 'Have you ever seen a jellyfish with legs? Honestly, Stanley, get a grip.'

'The jellyfish is the only one without a spine,' sulked Purbrick.

'Trust you to champion the one creature that has no backbone.' She patted his hand.

'The jellyfish is an oriental delicacy.' He puffed defensively. 'You cook it until it has a consistency you can squish through your teeth.'

'That's a tad more than I need to know,' said Maggie. 'Drink your tea and take a nap, dear.'

The railings were not high, but they were sharpened to an array of severe points. In addition, several cameras were visible through the undergrowth, and what appeared to be a guard post stood inside the low white-framed entrance. Vince decided that it would be best to get nearer to the buffalo enclosure from the outside of the park, rather than risk running through the centre of the gardens accompanied by the hooting of disturbed orang-utans. Luckily, a series of useful maps mounted inside the railings pointed him in the right direction. At the nearest possible point to the bison and buffalo enclosure, he painfully shunted himself over the black iron fence.

He held fond memories of visiting the animals as a child, but a return visit last year had proven a depressing experience. Many of the display cages in the almost-bankrupt zoo had been boarded up, their animals dispersed. Paint peeled from the parrot house, where dejected birds now concealed their once-radiant plumage as if ashamed of their diminished circumstances, and there had been a man selling encyclopedias in the gloomy calm of the aquarium. Indeed, the place seemed torn between a desire to present itself as an ecology centre and the need to make money. In the distance Vince could see a carnival-yellow bouncy castle and fast-food kiosks lining the once-grand central square. The schizophrenic nature of the place had been summed up by the fact that the tiger pelts and alligator handbags confiscated from smugglers and displayed beside cages as examples of callous commercialism had been forced to carry 'Not For Sale' tags.

If there were any buffalo outside tonight, sleeping beneath the dripping trees, he certainly could not see them. The pathways stretching off between the enclosures were rainswept and deserted. He clambered over the low iron fence and walked through the wet straw-strewn pasture. Presumably the animals were kept in on a night like tonight, when the overhead storm might panic them. He reached the holding pens, but there were padlocks fastened on the doors.

He folded open the mobile phone and punched out Masters's number, confident that his voice would be concealed by the noise of the rain in the trees. 'I can't see anything here for me,' he told the doctor. 'There's nothing in the exterior section of the enclosure, and the rest of the place is locked up tight. Are you sure I'm meant to be heading for the buffaloes?'

'Well, no,' Masters admitted. 'But nothing else really strikes us as the odd man out.'

'What about the penguins?' he offered. 'They're the only creatures on the list that are unable to fulfil one of the main functions of their species.'

'Oh, I see what you mean – they can't fly, can they? But why would that single them out for attention?'

Vince thought for a moment. He tried to recall his half-drunk conversations with Sebastian in the elegant restaurants they had frequented as friends. All those class-comparison lists they had made together; songs, schools, painters, architects, writers, pastimes – no animals had been mentioned, though he remembered Sebastian's sharp little denigrations of his heroes (Albert Camus 'too lefty-liberal'), and the admiration he had expressed for his own idols (Albert Speer 'a misguided visionary'). But why would he have mentioned Decimus Burton in the clue? Why name an architect? As the answer descended upon him, he could not help but chuckle at the crafty little paradox Sebastian had presented.

'Vince, are you still there?' asked Masters, alarmed.

'Yes,' he replied. 'I think I detect the hand of the author in this challenge. It's one of Sebastian's own. And it's not about the animals at all. It's about the zoo.' He stepped out from the eave of the barn and headed back towards the edge of the enclosure.

'What do you mean?'

'Sebastian and I have very different heroes. I expressed an admiration for Berthold Lubetkin, the great social architect who once said "Nothing is too good for ordinary people". Sebastian violently disagreed with me.'

'I'm sorry, I don't see the connection.'

'Lubetkin designed a masterpiece for the London Zoo. Hang on a second.' He climbed across the fence and dropped onto the concrete walkway ahead. 'During the last century this was one of the few private properties open to the public that truly cut across class distinctions. It was where the proletariat came to promenade. Its very name came from a popular music hall song. And in 1936, Lubetkin built a penguin pool for the zoo. Don't you see? It's Sebastian's comment on his perceived failure of such high ideals. A brilliant social designer and humanitarian is now solely remembered for a building that houses flightless birds.'

The white oval of the sunken pool, dazzling even in rain and darkness, was in sight. He rang off and sprinted along the edges of the path until he was forced out into an open concourse. The pounding rainstorm had at least driven any patrolling security guards back into their offices. Vince ran up to the edge and peered in. A handful of bedraggled penguins stood around the lip of the cobalt pool, sheltering from the downpour. Across the centre, two sweeping white ramps curled around each other in an elegant descent to water level. On the top one stood a figure dressed in black and white motorcycle leathers, holding a pale envelope. He held the envelope high. 'If you want to capture the last challenge, Mister Reynolds, you'll have to take it from me.'

Vince was exhausted. The thought of climbing into the penguin pool and having a fist-fight with a complete stranger was not one which appealed, but he seemed to have no choice in the matter. Setting his duffel bag against the wall, he searched for a way down. He would have to climb onto the same ramp occupied by his challenger, and it looked too fragile to support one man, let alone two.