Max and Autumn pedalled as hard as they could up the underground driveway towards the exit on John Islip Street. The place was a feat of engineering – part of the rebuild after the bomb.

At the end of the tunnel, a single roller door opened onto the rear of a parking garage just wide enough to accommodate a turntable. Another pair of hinged doors shielded that space from the street – it was completely innocuous, apart from the fact that garages in this part of London were something of a rarity.

Max turned and saw Lawrence behind them.

‘Faster, Autumn,’ he cried as they reached the garage door. It opened as they passed the sensor.

The children sped out onto the street and the doors began to fold in behind them.

But they would open again soon, and the man who was trying to kill them would be on their tails.

‘This way!’ Max shouted. He was planning to take the most direct route to the park where Magoo was meeting Tippie when the sight of his sister and Curtis running down Ponsonby Terrace towards their house diverted his attention. He skidded and turned right, Autumn behind him.

‘Kensy!’ Max yelled.

She immediately stopped and spun around. ‘Max! It’s Tippie! She’s behind everything. And she’s planning to kill us.’

Max stopped. ‘Get on!’ he told her. Kensy climbed up onto the seat and Max stood on the pedals.

Curtis jumped on the bike behind Autumn.

‘No time to talk. Magoo is meeting Tippie at Parliament Square in ten minutes and we need to shake that guy too,’ Max explained.

‘That’s Lawrence MacGregor – also known as Michael Smith,’ Kensy said.

‘We know,’ Max said.

A bullet whizzed above their heads.

Kensy turned back to look at their assailant then yelled at her brother, ‘Have you got anything in this backpack?’

‘Loads of stuff.’

She pulled out a stun grenade and removed the pin, then hurled it over Autumn and Curtis towards Lawrence. It went off, unsettling him, but after the smoke cleared he was still coming.

Max took a left turn along the river. He was pedalling as hard as he could while Kensy rummaged around, looking for something else to use. Behind them, Curtis and Autumn were far more vulnerable – they weren’t armed at all.

Another bullet narrowly missed their heads.

‘Tippie told Magoo you and Curtis had gastro,’ Max yelled at his sister.

‘The lying cow!’ Kensy shouted back as they flew along the path, dodging joggers and dog walkers.

There were increasingly more people on the street. Lawrence had put the gun away for the moment, but he was still on their tails.

‘She said you’d found Mim and she was looking after you both,’ Max said.

‘That’s a joke,’ Kensy said. ‘She drugged us and left us for dead in the games room. Fortunately there isn’t a lock I’ve met that I haven’t been able to pick.’

‘How did you get here?’ Max asked.

‘We took that prototype chopper – the one Mrs Vanden Boom had been working on. Almost made it, except that we ran out of fuel – but at least we didn’t crash. We had to catch a train after that or we’d have been here earlier,’ Kensy explained.

She turned around to see Lawrence was gaining on Autumn and Curtis. She spotted her drones in the bottom of Max’s bag. Kensy fumbled while taking Ferdinand the bee from his case. She put her glasses on, then launched him from the back of the bike. It was hard to balance on the seat without holding onto her brother, but that was the only way she could fly the drone.

Kensy raised the bee into the air and flew him straight at Lawrence until she could see his face up close. Now to land it on the target.

She manoeuvred the device behind the man, then set Ferdinand down on his neck, just below his hairline. As she detonated the stinger, she saw Lawrence take a hand off the bike and slap at it. Hopefully she’d unloaded enough poison to have some effect.

‘I think I got him,’ she said, dodging the man’s hand again and flying Ferdinand back towards them.

‘How long will it take?’ Max asked.

Kensy wasn’t sure, but if it was the direct hit she thought it was, then maybe a couple of minutes. Max neared Westminster Abbey. They could see Parliament Square Garden ahead in the distance. Behind them, Lawrence was starting to wobble. Curtis and Autumn could see it too. The man was soon swerving across the road.

Curtis circled back and rode up alongside him. Once they were within reach, Autumn leaned out and pushed him off the bike. Lawrence toppled onto a raised patch of lawn where a large statue of Abraham Lincoln stood proudly atop a stone plinth.

The children were on him like a flash, pulling him behind the statue and removing the gun, which Curtis dropped into his backpack. Autumn stood over Lawrence, hoping that they wouldn’t suddenly be rushed by some good Samaritans who had seen the takedown, but they were shielded by the statue. Now they were stopped, it just looked like they were taking a break from their family ride.

Autumn pulled her Pharos-issue Swiss Army knife from her pocket and removed a length of copper wire, using it to truss the man’s hands behind him. Then, with Curtis’s help, she pulled him up against the plinth. Lawrence looked as if he were having a nap.

Kensy and Max rode back to see if the pair needed any help. ‘Call Reff – he’ll come and take care of things,’ Max said to Autumn, who had already pulled out her phone.

‘Can you see Tippie and Magoo?’ she asked Max. He was scanning the park across the road with the magnifying device on his glasses.

‘There!’ He pointed towards the statue of Winston Churchill. Magoo was standing on his own, looking in every direction.

‘There’s Tippie!’ Kensy pointed at a woman walking across from Great George Street.

‘What now?’ Curtis asked.

‘We need to hear to what they’re saying. Kensy, we packed all the drones. Which one has the best listening device?’ Max asked.

‘Roger,’ she answered. Max pulled out the small metal box holding the creature and within a few seconds, Kensy flew the dragonfly towards the couple. She hovered above the MacGregors.

‘What are we doing here, Magoo?’ Kensy heard Tippie say. She sounded very testy.

‘I know what you’re doing,’ he said. ‘And you have to stop. It’s not right. Cordelia is family.’

Kensy relayed the conversation to the others.

‘So he is on our side?’ Autumn said.

‘Seems so,’ Max said.

‘I’m just taking what’s yours, Magoo,’ the woman said. ‘You’re too soft to do it yourself.’

‘But you promised that you weren’t going to do anything. I told you I didn’t want it and I meant it. How could you betray me?’ Magoo said.

Kensy was incredulous. The others badgered her to share, but she held her hand up and listened some more.

Max watched Magoo’s and Tippie’s body language. She stood rigid – clearly angry – while Magoo was leaning forward – almost as if he was begging.

‘Oh no! She can’t!’ Kensy shouted.

‘She can’t what?’ Max demanded, but his sister was already running across the road towards the pair. Max sprinted after her and Curtis too. Autumn stayed behind – someone had to keep an eye on Lawrence, whose eyelids were beginning to flicker.

Kensy saw Tippie pull a pen from her pocket. She knew immediately it wasn’t your average biro.

‘Mr MacGregor!’ the girl shouted. Tippie turned and saw her flying towards them. But it was too late. The woman had already fired. Her husband looked at his arm, where the projectile had pierced his skin.

‘Tippie!’ he gasped. ‘How could you? I love you.’

The man collapsed onto the ground.

Kensy had almost reached the pair. Tippie didn’t have enough time to reload. The woman took off running towards Big Ben.

‘Time’s up, Tippie!’ Max shouted. ‘Stop!’

But she didn’t.

‘Tippie!’ Magoo shouted weakly as his wife raced towards the road.

But Max was one step ahead of the woman. ‘Oh no, you don’t!’ the boy yelled as he pulled a shoelace from his pocket and twirled it like a lasso before releasing it into the air. The gadget flew at Tippie, wrapping around her ankles.

She fell heavily, cracking her head against the road – right in front of an oncoming taxi.

‘Tippie!’ Magoo wheezed as the poison took hold and he slumped down onto the grass.

The vehicle screeched to a halt.

Kensy looked at the driver and couldn’t believe her eyes.

‘Shugs,’ she breathed.

He leapt from the vehicle and ran to Tippie’s side.

‘Not you!’ Kensy spat. ‘You’re not rescuing her.’

Shugs shook his head. ‘I came after her,’ the man said. ‘She’s got to be stopped.’

A crowd was starting to form. Tippie was deathly still and bleeding from the back of her head, her eyes closed. People had their phones out, presumably calling for help.

‘You’re lying. You tried to kill Mrs Thornthwaite,’ Kensy said, checking the woman’s pulse. It was still strong.

Shugs was pleading. ‘No, it’s not true. I didn’t lay a finger on Ida, I promise. I can explain everything.’

Kensy looked at the man, unsure of whether to believe him.

‘I swear,’ Shugs said. ‘And the only reason I took those shots at the helicopter was because I thought it was Tippie and Michael – then when the chopper spun around I saw it was you two . . . Tippie’s a monster.’

Curtis nodded. ‘He did stop shooting – I think he’s telling the truth.’

‘How did you even get here?’ Kensy eyeballed the man.

‘You’re not the only one with flying skills, miss,’ the man replied.

‘Fine,’ Kensy yelled. ‘Shugs, you get her head and we’ll take her legs.’

‘Shouldn’t you wait for an ambulance?’ a well-meaning woman in a red suit asked.

‘No time,’ Kensy said as the three of them bundled Tippie into the back of the cab. Kensy and Curtis began tying the woman up while Kensy shouted at Shugs to drive.

Max was busy looking after Mr MacGregor. The headmaster, too, was unconscious, but if Max could extract the poisoned pellet before it completely released its contents, Magoo still had a chance. Unfortunately, some onlookers had turned their attention towards the boy and were heading towards him. He had to act quickly.

Max pulled out his own Swiss Army knife and gritted his teeth, digging into Magoo’s skin. The pellet had gone deeper than he’d hoped. Once he could see it, he used the tweezers contained within the unit to extract the projectile, which he wrapped in his clean handkerchief and dropped into his pocket. The wound was deep and would need stitches, but they could deal with that later. Fortunately there wasn’t too much blood.

Someone must have called an ambulance. He could hear the sirens getting closer.

Over by the Lincoln statue, Elliot Frizzle and Autumn were dealing with Lawrence while Monty Reffell went to Max’s aid, pushing through the onlookers to help the boy lift Magoo to his feet. The man had already called another cab. Before the paramedics arrived Magoo, Monty and Max were on their way to the Pharos Clinic and Tippie’s coup was over.