Snape is Chief Inspector at Scotland Yard and Boyle is his assistant. Nick doesn't like them but they are impressed by him after he solved the case of the Falcon's Malteser. Snape tells Nick about a convicted criminal called Johnny Powers, who is serving 15 years in a maximum security centre after he was caught trying to steal £1million worth of designer clothes from Harrods. Snape has recently found out about a mysterious criminal called 'the Fence' who buys and sells all the stolen goods in England, and would have traded in the designer clothes that Powers stole if he hadn't been caught. They already tried asking Powers who the Fence really is, and even offered to halve his sentence in return for a name. But Powers refused because he hates policemen. Snape then tells Nick that he wants him to go to Strangeday Hall - the maximum security centre where Powers is imprisoned - and share a cell with Powers in hope of finding out the Fence's identity. As soon as Nick finds out who the Fence really was, then Snape and Boyle will have him released straight away.
Nick goes home, where he lives with his older brother Tim, a bird-brained private detective. They have been living on the breadline recently, but Tim has just been paid by the British Museum for a new job. A ming vase called the Purple Peacock has been stolen and the Museum staff are so humiliated that they won't go to the police, so they went to Tim instead. It had been in the back of a van and on its way to be cleaned when the van driver stopped at a petrol station in Camden to refuel. When the driver had paid for the petrol, he returned to the petrol pumps and found the van gone - with the vase inside. Tim has been employed to find it.
With so much money behind him from the Purple Peacock mission, Tim finds £5 to pay for Nick's upcoming trip to Woburn Abbey.
On the trip to Woburn Abbey, Nick is walking out of the Woburn Carbuncles exhibition room when he hears a shattering of glass and an alarm bell ringing. He realises that someone has stolen one of the carbuncles and placed it in his pocket. The police arrive in no time and Nick tries to escape through the house, demolishing a host of expensive plates and knocking an old lady unconscious because she was attacking him with two needles and an expensive fallen painting. He then hides under the tarpaulin on the roof of a Land Rover which is parked on the visitors car park.
The Land Rover starts moving slowly and when Nick thinks its safe he gets out, only to find himself in the Woburn Abbey Safari Park. He comes face to face with a lion and fears for his life. The safari park warden is approaching in his jeep but Nick doesn't think it can reach him in time. He escapes being eaten by catapulting the Woburn Carbuncle into the lion's mouth with a slingshot, knocking it unconscious. An anaesthic dart is fired by the safari park warden and Nick faints. He realizes that the safari park warden had been aiming for him and not the lion all along.
Nick was locked up at Strange Day Hall and shared a cell with Johnny Powers. Soon after his arrival, he got a visit from Snape and Boyle who told him that they framed him and reminded him that he must try and find out who the Fence is, so he can come out of prison soon. Nick protests that he had turned down Snape and Boyle's offer when they came to visit him at school, but Snape replied that there was nobody else to do the job so they set up Nick for the Woburn Carbuncles robbery anyway.
A few weeks into his sentence, Nick is entering the prison showers when he sees Johnny Powers from a distance. He has been beaten up by three other inmates - Zuckie Hommel, Mark White and Mark Hards. Mark White has a gun and vows to shoot Johnny in two minutes when Concorde flies over, so the noise of Concorde will drown out the gunshot.
Nick saves Johnny by messing with the pressure gauges for the shower. The other inmates are injured but survive. The gun is retrieved without the staff's knowledge.
The day after the incident in the showers, Johnny receives a coded letter from his mother. She is telling him that he must escape soon because a gangster called Big Ed is taking over his territory in London - Big Ed was the man who had somehow sent a gun into Strangeday Hall and tried to have Johnny murdered. Johnny then tells Nick that he must escape too, and the day before the escape Johnny's getaway driver Nails Nathan became ill with food poisoning. So Johnny tells Nick that Tim has to be the getaway driver instead.
Nick gets a visit from Tim later that day and tells him to go to the departure lounge at Heathrow Airport at 11.00pm - that was where Ma Powers, Johnny's mother, would be waiting with the getaway car.
At midnight, Johnny tells Nick that it's time to escape. They pistol whip a guard and end up jumping onto a modified truck driven by Ma Powers.
Tim's erratic driving helps offset four pursuing police cars and forces a lorry full of eggs to swerve off the road, while Ma Powers gave him a bit of help with her machine gun. Then a fifth and final police cars catches up with them and Nick recognises the two back seat passengers as Snape and Boyle. Ma Powers opens fire and the car's windscreen shatters. The car swerves into the kerb, spins through the air and hits a telephone box before exploding. Nick is convinced that Snape and Boyle are dead - and they had been his only way out of this mess.
The people mover then loses control on a bend and the overcarriage crashes through an empty office block and lands on the River Thames. They float along the river and climb out of the people mover at Wapping.
Johnny Powers and his mother are living in a tumbledown old terraced house in a rundown Wapping street. It is three stories high, except the third story has fallen in on itself. Johnny's sidekick Nails Nathan is living with them.
Nick, Tim, Johnny, Ma Powers and Nails Nathan learn via the television the police are looking for Powers and Diamond in relation to the escape from Strangeday Hall. Johnny had been dubbed 'Public Enemy Number One' after his armed robbery conviction, but now the media were considering Nick to be 'Public Enemy Number Two'. The police wish to speak with Nick's brother, Tim.
Nick orders Tim to behave like a gangster, if his cover is blown he will be killed.
Nick later overhears Johnny telling his mother that he is going to Penelope. He is convinced that 'Penelope' is the Fence, and decides to follow him. Johnny had told Ma Powers to make sure that nobody left the house while he was gone, so Nick has to be very careful. Nick follows Johnny through the dilapidated streets of Wapping until he reaches Wapping Tube Station and wonders why Johnny be going there. He thinks that Johnny could be meeting 'Penelope' on the platform, or he could be catching a train to wherever she is. But Johnny suddenly vanishes and Nick can't find him, despite searching every nook and cranny of the station. Nick catches the elevator back to street level. The next thing he knows, a car is stopping and two people are getting out of the car and forcing him in.
Nick has been kidnapped by three men - a punk and two men in women's clothes - who take him to Clapham Junction. They lock him in an empty carriage for about two hours before the punk lets him out. Nick follows the punk into another carriage where a man called Big Ed lives.
Big Ed introduces himself as Zuckie Hommel's uncle. He is angry with Nick for stopping Zuckie and the two other men from killing Johnny Powers - by causing the explosion in which Zuckie was badly burnt. When Big Ed found out that Powers had escaped, he had sent his cronies out to catch him but they had captured Nick instead. Big Ed then orders his cronies to take Nick away and kill him. They tie him to the railway track, knowing that an Intercity Express train will soon be passing through Clapham Junction.
Just after Big Ed's henchmen leave, a man approaches Nick and cuts one of his hands free and helps him from the tracks just before the train passes. Nick is convinced that he knows the man from somewhere but can't put a name to his face.
Nick finds a box of matches on the platform at the nearby railway station and empties an oil drum within Big Ed's compound. He goes to the nearest telephone box and asks for the police, who are interested in finding Big Ed. Nick tells them that Big Ed is living in carriages near Clapham Junction. He tells them that it won't be difficult to find Big Ed's hideout and proves this by dropping a match on the pool of oil that surrounds the carriages. The fire quickly spreads and Nick walks away, convinced that Big Ed and his cronies will be burnt to death.
Nick decides that he must go back to Johnny Powers and Tim, as they will surely be wondering where is by now. And he is still determined to find the Fence, although with Snape and Boyle dead he didn't think anyone would believe his story. Then he realises that Palis could have seen Snape and Boyle on the afternoon that he was serving a detention. He heads for Palis's flat in Chelsea but is spotted by the police just before he gets there.
Nick hides on the roof of a boiler room outside the apartment block where Palis lives. The police appear on the pavement and are speaking in such loud voices that Palis is roused and appears on his balcony. He sees Nick but tells the police to go looking somewhere else for him because he obviously isn't here.
When the police are gone, Palis tells Nick to come into his flat and he explains the whole story. He did not steal the Woburn Carbuncles, and had been working for the police all along. Nick then asks Palis if he had seen Snape and Boyle, but he hadn't. Palis lets Nick sleep in his spare bedroom for the night but urges him to back to Johnny Powers and Tim the next day.
Palis drives Nick back to Wapping the following morning and tells him to get in touch if he needs anything. At the hideout, Nick sees a doorbell. Not recalling one, he sneaks into the house and rescues Tim from a bomb rigged to go off if the newly-installed bell had been rung.
Ma Powers had left some food in the kitchen and serves lunch for himself and Tim. Tim then explains that Johnny had come back the previous afternoon from wherever he'd been to find Nick gone. He'd asked Tim questions and was suspicious about the answers. The atmosphere during dinner had been as frozen as the fish fingers Nails Nathan had served. Tim had gone to bed early only to be dragged out of dawn. Before Johnny left, he said that they'd have gone underground by the time Nick got back. Nick is still determined to find out who the Fence is, so they spend all afternoon scouring Wapping for anywhere that might lead them to the Fence. Nick then notices a houseboat called 'the Penelope', moored to the quay. He realises now that 'Penelope' is not the name of the Fence, but it is more likely to be the meeting place where Johnny does his dealings. It is surrounded by a high fence which is impossible to climb over, so Nick and Tim manage to swim to the other side. They are disappointed to find the houseboat empty and Nick thinks that the Penelope is not behaving like a boat should be.
Nick and Tim decide to keep a watch on the Penelope from a nearby derelict house. They wait six hours before they see anything. A light goes on in the Penelope and a man appears. He is carrying a crate and before long four more men carrying crates join him. A lorry then turns up and someone unlocks the gate before loading the crates into the back of the lorry. Two more lorries are then loaded with goods including a piano and a hi-fi - all of which come out of the Penelope. And when Nick and Tim had been inside it was completely empty. Nick then realises that the Penelope hadn't titled when he leaned against it, and had resisted the choppy water in the Thames. He remembers that Johnny 'went to Penelope' through Wapping Tube Station.
The five men disappear into the lorry and back into 'the Penelope'. Nick and Tim head for Wapping Tube Station. The Tube system was closed for the night, but the lights were still on and the entrance was still open. They searched the station thoroughly but still couldn't find anywhere that might lead to the Fence's headquarters. Tim then turned on a tap in a store room because he was thirsty, and a panel in the wall suddenly opened. This was obviously where Johnny had 'vanished' to when Nick had followed him.
Soon after Nick and Tim entered the tunnel, Tim sneezed. A door opened at the other end of the corridor and they could hear Johnny Powers talking to his mother. Johnny had heard sneezing but Ma Powers assured him that he hadn't heard anything. Then the door closed and Nick and Tim proceeded further into the complex. They soon found themselves in a network of passages which were crammed with stolen goods including fur coats, cutlery, hi-fis and televisions. Tim then found the Purple Peacock, which he was being paid to recover. Then Nails Nathan appeared and was holding a gun, but Nick pinned him to the floor and shouted for Tim to hit Nails. But Tim couldn't bring himself to smashing the vase and Nails fired a bullet from the gun, shattering a mirror and drawing more armed men to the scene. Then Johnny Powers appeared with his mother and ordered Nails to tie Nick and Tim up before locking them in another room.
Nick had brought the bomb with him in his backpack, as he thought it might be useful to him later. It was attached to the Mickey Mouse alarm clock, and Nick set it so it would explode and blow the locked door off. He was confident that they could both escape in the confusion.
The story ended how it had begun - with Nick being in Mr Palis's French lesson on a hot afternoon. Mr. Palis was challenging the class with French phrases which they then had to translate into English. Mr. Palis told Nick to read the next sentence. Nick translated some French text and suddenly starts to feel ill. He then realizes that he'd just read something on the blackboard which was horribly wrong - or maybe just horrible. He focuses on the text that he'd just read out. He finds that he has said "someone called from over the fence." and realizes that he'd just said - the Fence. Palissade.
Nick stares at Palis and soon works out that Palis is the Fence. It couldn't have been anybody else.
When Snape had first told Nick about the Fence, he said that the Fence could have been hiding behind any ordinary occupation - a banker or a shopkeeper, for example. He could also have been a teacher. And with his afternoons off and long holidays, Palis would have had plenty of time to concentrate on his other career. And Snape had probably suspected Palis from the start. Otherwise, why would Snape have chosen Nick of all people to go to Strangeday Hall? He'd guessed that Powers would escape and Nick would escape with him. And when Powers escaped, he would soon meet the Fence and if Nick was there too, he would recognise him.
Nick had wondered how Palis knew so much about antiques and valuable items during the Woburn Abbey trip. As the Fence, that was his profession - buying and selling stolen goods of all shapes, sizes and ages. When he had stayed at Palis's flat, he had seen what he thought were copies of Rubens and Picasso on the walls. Now he knew that they were the real thing. As soon as he'd gone in there, he'd blown it. Palis had rescued Nick from the police because he thought he was on his side, and had been about to tell him that he was the Fence. But before Palis could speak, Nick had said that he was working for the police and that he had broken out of Strangeday Hall with Johnny Powers. So Palis had contacted Johnny Powers and organised the bomb plot which had nearly blown Nick and Tim up.
Palis surely knew that his days were now numbered.
At the end of the lesson, Palis announced to the class that he was leaving. He dismissed the whole class except Nick, who knew exactly what Palis was planning. As Palis was producing a gun, Nick fled the classroom and dodged a shot from Palis's gun, which instead smashed into the door frame. He dodged a second bullet which injured a teacher called Mr Roberts instead, who exited his classroom. Nick ran upstairs and hid under a workbench in a laboratory. The Fence saw him go in and shot him in the arm, but Nick managed to smash a bottle full of chemical in his face, damaging Palis's eye. He then ran onto the roof of the school and below him police cars were surrounding the school. Snape was there too. He was late, but at least he'd arrived.
Palis then appeared on the roof, clutching his smoking eye, and shouted at Nick, saying how he had destroyed his life's work. Nick responded by telling him that it was 'his last post'. Palis conceded this, but said he would have some satisfaction from the fact that he'd be taking Nick with him....
Palis squeezed the trigger of his gun, but had run out of bullets. He ran at Nick like a wild bull. Nick stepped aside. Palis plummeted down screaming and within a few seconds the screams had stopped. The Fence had impaled himself on a fence.
With Palis dead, the story ended with Nick's troubles over. His main target had been uncovered and was now dead. Johnny Powers and Nails Nathan were either dead or in Japan. The rest of the gang were safely behind bars. The fate of Big Ed and his gang is never revealed, but they were surely burnt to death in the inferno at Clapham Junction before they could get away.