Fizz ate breakfast early the next morning. He was too excited to sleep. Sergeant Stern had called everyone to the agility course for their first training session and he didn’t want to be late.

‘Wait for me!’ called Remi, racing to catch him. ‘Did you hear what happened to Wesley last night? Someone swapped his food bowl and he had fried liver instead of boiled chicken. He had an allergic reaction and had to go home. The cook’s furious. She’d bought four weeks’ worth of food especially for him. That’s two dogs who have been sent home and we haven’t even had our first training session!’

‘You mean three dogs,’ said Fizz.

‘Someone stole Lucinda’s squeaky toy last night. She was so frantic Sergeant Stern told her to go home. He had thought she was good enough to take out the Top Dog Award this year.’

‘Out of my way, losers,’ said Amadeus, shoving past Fizz and Remi. ‘The only Top Dog around here is me!’

Fizz and Remi joined Sergeant Stern and the other trainee police dogs in front of the agility course, which was a track laid out with obstacles along it.

‘This morning we’re going to work on your coordination, strength, and stamina,’ he said.

Fizz’s heart pounded in his chest. The agility course looked challenging.

‘Fizz will go first, followed by Apollo,’ said Sergeant Stern. ‘Take care on the balance beam, it’s trickier than it looks.’

Fizz stepped up to the starting line. He could see the high balance beam in the centre of the course.

‘Ready?’ asked Sergeant Stern. Fizz nodded. ‘Go!’

Fizz sprang over the first two jumps, then weaved in and out of a set of posts, sending clumps of grass flying. He raced through a cloth tunnel, jumped through a set of hoops, and ran up and over a seesaw. His muscles tensed. The balance beam was ahead. He climbed up onto the high platform and paused.

‘He’s chicken. Too scaredy to cross,’ jeered Amadeus.

Fizz’s heart raced, but he stepped carefully onto the balance beam. The beam wobbled under his feet. Fizz caught his breath, then tiptoed step by step across the shaky beam.

‘You made it, Fizz!’ cheered Remi, standing on her back legs and stepping in a circle.

Fizz raced down the ramp, scrambled over a set of slippery barrels, and skidded down a slide to the finish line, in front of Sergeant Stern.

‘Good work, Fizz,’ said Sergeant Stern.

Apollo the Doberman was next. Fizz watched as he bounded onto the balance beam and took some wobbly steps.

‘He’s going to topple,’ cried Amadeus. ‘For sure!’

The balance beam shook and quivered.

‘Come on Apollo, you can do it!’ barked Remi.

Then, with a crack and a yelp, Apollo and the balance beam crashed to the ground.

‘He’s hurt!’ called Fizz, running over to help. ‘Look, he’s twisted his ankle.’

Sergeant Stern inspected Apollo’s leg and the broken balance beam. ‘It looks like you’ll be going home, Apollo. Your ankle is too swollen to continue training.’

Sergeant Stern helped Apollo to his feet. ‘Amadeus, walk Apollo slowly to the vet clinic so Ms Jabb can strap his leg. Sorry, everyone, but that’s it for the agility course today. We’ll continue your stamina training on the exercise tracks and swimming pool. Fizz and Remi, I’d like you to stay behind for some undercover training please.’

Fizz’s legs shook like jelly. Remi bounded over enthusiastically.

Sergeant Stern clasped his hands behind his back. ‘Every year, the undercover dogs are given a special challenge to test their detective skills. Fizz, Remi, your challenge is to solve a mystery. Four trainee police dogs have been sent home and replaced with dogs from the waiting list. I don’t believe they’re all accidents. I want you to find out who’s doing this and why. Solve the mystery, and you’ll pass your undercover training. Fail, and you won’t graduate to become undercover police dogs.’

Fizz gulped.

‘Start by investigating the balance beam,’ said Sergeant Stern, before he turned and headed off for the exercise tracks.

Fizz and Remi inspected the balance beam together.

‘Look,’ said Remi. ‘Someone’s chewed the bottom of the balance beam support, and they’ve dug around the base to make sure it would topple.’

‘Sergeant Stern’s right,’ said Fizz, narrowing his eyes. ‘This is no accident. This is sabotage! Someone switched Wesley’s food bowl on purpose, someone hid Lucinda’s squeaky toy, and someone sabotaged the agility equipment today.’

‘But who?’ asked Remi.

‘Who has the most to gain if the best dogs are sent home?’ asked Fizz. ‘Who has to win Top Dog or else? Amadeus!’