Two months had passed since the snack bar case, and Thomas Monroe and his daughter/accomplice, Alicia, were in jail. Mason was no longer grounded. He was downstairs watching TV when Mackenzie came down with a list of things for him to review that could help improve their business and make them stand out more. "Hey Mason," said Mackenzie, "I was wondering if you could take a look at this list I came up with. It might help improve the business."

"Let me see it," said Mason. Mackenzie handed the list over to Mason. "Our own office?" asked Mason. "Mom and dad won't even let us get a dog. You really think they're going to let us get our own office?"

"Ok, fair point," said Mackenzie. "What about the other stuff on the list?"

"You want weapons?" asked Mason. "You can't be serious. Mom and dad would freak if they found out we bought a gun, and I really don't want to be grounded again."

"Once again, you make a good point," said Mackenzie. "Continue."

Mason read further down the list. "Why do we need a detective's kit if we're already detectives? Aren't those for people who are just starting out?"

Before Mackenzie could respond, the phone rang. "Mystery Twins Detective Agency, this is Mackenzie speaking," she answered. On the other end was a little girl named Olivia who was calling about a lost puppy. "We'll be right there," said Mackenzie. "The list will have to wait; someone needs our help."

Mason was hesitant over what happened last time but decided to go along anyway. "This isn't another trap, is it?" he asked.

"I have no idea," said Mackenzie, "but in case it is, we'll be extra careful this time."

The twins left the house and started heading to the address given to them by Olivia. "1065 Waker Street, this is the place," said Mackenzie as they walked up to the front door.

When the doorbell rang, it wasn’t Olivia who answered; instead, her father, a well-dressed man who seemed to be in a hurry, opened the door. Mackenzie introduced herself and her brother Mason as the Mystery Twins and inquired about Olivia and the lost dog. “I'm already late for work, but Olivia is downstairs in the basement. Just try not to get into too much trouble, okay?” Olivia's father responded with a stern look.

In the basement, Olivia told the Mystery Twins about her lost dog named Buddy, a dachshund-beagle mix, white with a brown spot on his back. She explained that he went missing while they were playing in the park. The only place he likes to go is the park, and all she had of him was a picture. The Mystery Twins decided to go to the park to investigate.

At the park, Mackenzie asked Olivia to point out where the picture was taken. It was near the playground, and Olivia revealed that she had taken the picture herself and that it was from a Sunday afternoon. Mason noticed something unusual in the picture and asked Olivia about the ice cream trucks operating on Sundays. Olivia confirmed that they typically only operate on weekdays. This prompted the Mystery Twins to head towards the street, suspecting that the ice cream truck's presence might have something to do with Buddy running off.

The three of them proceeded to head in the direction Olivia’s dog ran to when Mackenzie stopped and picked something up off the ground. "A dog whistle," she said. "I knew it. Whoever uses this can control dogs by using a frequency only they can hear. I suspect that somebody used this whistle to call Buddy and lure him away from the park." Mackenzie looked over the whistle some more until she noticed something. "There seems to be an address inscribed on the back of this," she said. "Pet Heaven Animal Shelter, 765 Mayfair Ave. I think we need to try and find this address; it might lead us to whoever is behind this," said Mason. "Good idea," said Mackenzie. "Let’s head over there."

When they arrived at the animal shelter, they approached the desk and introduced themselves to the woman behind it.

"May I help you?" she asked politely.

"We were wondering if you could tell us about this dog whistle we found," said Mackenzie.

"That’s one of our training whistles. Wherever did you find this?" the lady asked, happy to have it back.

"We found it on the street not far from here," said Mason.

"Do you recognize the truck in this photo?" asked Mackenzie.

"Why, yes," said the lady, "that’s one of the trucks we use to pick up stray animals and bring them back here, but why is it disguised as an ice cream truck?"

"We were hoping you could tell us," said Mason.

"Do all of your employees have access to these trucks?" asked Mackenzie.

"We don’t lend them out to new hires for safety reasons," said the lady, "and the ones who do have access are only supposed to go on a specific day and time of the week. Actually, just the other day, I noticed one was out on a day it wasn’t supposed to be."

Mackenzie thought for a moment. "Do you happen to know the day and time in which this happened?" she asked.

"I believe it was a Sunday afternoon," said the lady.

"Would you kids mind telling me what all of this is about?" asked the lady.

"It’s kind of a long story, but we’re trying to find a missing puppy, and I think your shelter might be at the center of it as I believe that someone is luring dogs away from their owners using a whistle," said Mackenzie.

"Do you keep a log of all of your employees?" asked Mackenzie.

"Of course," said the lady. "Here it is."

"Mason, take a look at this," said Mackenzie. There was a truck signed out twice in the past two days at the same time, signed by Jeremy Smith and Logan Langford. Mackenzie called the receptionist over to see if she could identify the names on the list. "I recognize the names of those men; they just started working here about two weeks ago, but they don't have permission to drive the trucks," Mackenzie started to think for a moment. "Do you give your employees keys to the building?" she asked. "No," said the receptionist. "The only ones who have keys to the building are me and the owner, Eric Thomson."

"I think we're getting close to solving this mystery," said Mackenzie. "Whenever you send a truck out to pick up strays, they are brought straight here, right?" "Yes, that's right," said the receptionist. "I have a suspicion that someone might be using your shelter as a front for illegal smuggling," said Mackenzie. "But in order to prove this theory, we're going to need to see what's in the back of the shelter." "Of course," said the receptionist, "right this way."

As the three kids headed to the back of the shelter, Olivia spotted a suspicious-looking elevator. "Has this always been here?" she asked. "No," said the receptionist, "it was just added a couple of weeks ago." Mason began to think. "Was it around the same time those two guys started working here?" he asked. "I think we have our culprits," said Mackenzie. "Let's head down this elevator and see what we find."

They all got into the elevator, and they noticed a button labeled "B." They pressed it, and the elevator started heading down until they arrived in a large bunker where hundreds of pets were being kept. Mackenzie started taking pictures for evidence, and Olivia started looking for any sign of her dog, Buddy. Eventually, she was able to locate him, but the door was locked with seemingly no way to open it.

It wasn’t long before they eventually started to hear footsteps. "Someone’s coming," Mackenzie whispered. "We need to find a place to hide until we can get out of here," Mason whispered. Before they could escape, they caught a glimpse of two men walking towards them, with one of them talking on the phone. "Logan and Jeremy, I presume," Mackenzie said quietly. "But who’s he talking to on the phone?" asked Mason quietly. "It’s gotta be their boss," Mackenzie said quietly. 

"Yes sir, the contraband is ready for pick up, and don’t worry, you’ll get your money in cash, paid in full," said the guy on the phone. "Is the plane on its way?" asked his partner. "Yep, should be here within the hour."

Everything was going according to plan until Mason felt a sneeze coming on. "Uh, guys, I think I need to—" "Mason, don’t," said Mackenzie quietly as she attempted to stop her brother from sneezing. "This is exactly why our parents won’t let us get a dog." 

Mason felt relieved as he thought the sneeze had passed until he let out a loud ACHOO, which ended up blowing their cover and getting them spotted. "Bless you," said Olivia. "Thanks," said Mason. "Hey, you kids, what are you doing down here? This is a restricted area."

Realizing they had been spotted, the three kids and the receptionist all started to run for it to evade capture. Mackenzie was almost caught, but she managed to slide under the guys' feet and escape. "It’s too late," she said. "We know everything about your plan. You won’t get away with this."

Meanwhile, Mason was busy with the other guy as he tried to run up the wall and do a flip, causing him to get knocked out while Olivia was trying to find out how to set all the animals free. Eventually, she decided to walk up to the guard who was knocked out and search him for a key, which she found along with an ID confirming that his name is Jeremy. She used the key to open all of the cages, including the one that held her dog Buddy.

They decided to tie Jeremy up with rope in case he wakes up and pull him off to the side. Then Mason goes to help his sister. He sees Logan running towards her and manages to trip him by spilling some dog food that he just so happened to find in a storage container, causing him to fall on his back. 

With the two men safely tied up, Mason hears the phone ring and decides to pick it up. On the other end was a man named Phil Johnson, and he was in fact the operator behind this whole thing and the boss of the two men. Eventually, the police arrived to take Logan and Jeremy to jail as well as wait for the plane to arrive and arrest everyone on board, including Phil Johnson.

"Sorry, Phil," said Mason, "but I’m afraid your smuggling operation has been foiled and your travel plans have been canceled." "And don’t even bother trying to talk your way out of this because we’ve got everything on tape," said Mackenzie. "Before we head home, let’s go see how Olivia is doing," said Mason .

The twins decided to check on their new friend before heading home.

"So, Olivia, how does it feel to finally have your dog back?" Mason inquired.

"I'm so happy to have him back; you have no idea. Thanks so much, guys," Olivia replied.

"Don't worry about it, Olivia; it's our job to help people like you. Ready to head home?" Mason suggested, but before they could leave, they were stopped by a couple of officers who wanted a statement about the events preceding their arrival.

Mackenzie explained that, as an investor, Phil would have access to all of the shelter's facilities, including owning a key to the building. He would give this key to his sons, Jeremy and Logan, who would go out during the day to capture dogs by luring them away from their owners and into the shelter's truck. Despite new hires not being allowed to drive the trucks, Logan and Jeremy had special privileges as the investor's kids. After capturing the dogs, they would take them back to the shelter after it had closed for the night and bring them to the bunker, where they would wait to be picked up by plane and sold on the black market for a profit.

"Well, thank you two. I really have no idea how you guys do it, but you've done it again," said the officer. "Do you three want a ride home?"

Mason gave Mackenzie a concerned look. "If mom and dad see us getting dropped off in a cop car, we're in big trouble."

"Yeah, you're right," Mackenzie agreed. "Thanks, but our parents would freak if they saw us being dropped off in a cop car. We'll just walk."

Before they started to head home, Mason stopped and asked where the receptionist went. "Hey, where did the receptionist go? She was just here a minute ago. You don't think she was involved in this somehow, do you?"

"Oh, don't worry about her," the officer reassured. "She's fine, a little shaken up after everything that happened but otherwise okay."

"Well, that's good to hear," Mason acknowledged. "Come on," Mackenzie said. "Let's go home."

As they were leaving, Mason inquired if they could accompany Olivia home, but she politely declined, assuring them that she would be fine on her own. After exchanging heartfelt farewells, they parted ways, and Olivia walked away, clearly delighted to have her dog back. “Olivia seems genuinely happy to have her dog back, don’t you think?” Mason remarked. “Absolutely,” Mackenzie replied. “Now, about that list…” “You’re still fixated on that ridiculous list?!” Mason exclaimed. “Just let it go already,” he added as they continued on their way home, fading into the distance.