Tails' Workshop
Tails and Nicole had made progress on designing the portal, now that they had the Chaos Emeralds as a power source. However, after drawing the plans up, they came to a sudden, unforeseen setback.
"The Emeralds need a special sort of harness to channel their energy," Tails explained to the others when they arrived. "Otherwise, the portal can't be controlled, and we could send Cosmo anywhere but her home dimension."
"Can't you just make one?" asked Maisie. "You've got plenty of metal lying around."
"Yes, but the Emeralds can't just be harnessed by steel or copper." Tails let out a frustrated groan before continuing. "The only metal that comes close to maintaining their power is honium."
That earned a frustrated groan from the rest of the team. "What's up? Is that rare or something?"
"'Rare' is an understatement, Maisie," replied Nicole. "Honium is a custom metal mixed from multiple different alloys that none of us knows how to make."
"Only one person knows the mix," added Sonic. "Eggman. All his bases were knocked down, and G.U.N. is keeping the remains under lock and key. How do you think we'll get those out?"
"Show the guards some leg and they might let you have some," teased Tails, earning a few chortles from the blue hedgehog.
The red rabbit then had an idea. "Or we could try Robotropolis." The others looked over. "I knocked over a tower when we were there, that could be enough honium to build five portals."
"That's a great idea," replied Nicole, her hologram disappearing momentarily as she began a new task. "I'll prepare some transport, and we can send a team over. Any volunteers?"
"I'll go, nobody knows that place better than I do," said Sonic at once. "What about you, Maisie? Wanna show me your axe skills?"
"It's a halberd," retorted Maisie. "And sure, I'll come. Wanna see what happened to our Eggman, Cosmo?"
"Sure thing," the Seedrian responded, the first time she had said anything since arriving.
"Hold on," interrupted Sonic, "the Tornado only has two seats. I could ride on the wing, but then how could Tails pilot?"
"Oh, you're not taking the Tornado," replied Tails, the fox gesturing out the window to the backyard. "This is a side project Nicole and I put together."
Maisie and Cosmo went agape as they saw a section of land opening up, the two bits of grass retracting as something rose up from the ground. What stood on the pad was a bulky looking craft with a pot attached to the bottom. Sonic was more confused, turning to his mechanic friend.
"You call that a side project?" he asked, earning a shrug from his friend. "I thought you'd be fixing up another Lotus."
"Introducing the Monsoon!" the fox declared. "I figured if we ever needed to carry around a lot of people or cargo, we could use this. Now we need it."
"Best of all, it can be remote piloted from here," added the holo-lynx, her hologram returning to close Maisie and Cosmo's jaws. "So while you guys go out and collect the honium, we'll be ready to melt it down. Does that sound sound?"
The others agreed, heading out to where the Monsoon was waiting. It did feel unusual, flying a plane with no pilot aboard, but they knew it wouldn't be that long. They felt a jolt as the aircraft took off, a look out the window showing the clouds pass by as the Monsoon lumbered along.
"It's gonna take us a while, so wake me when we get there, yeah?" said Sonic, the blue hedgehog falling asleep before either occupant could say anything.
Sonic was right, nobody knew Robotropolis better than he did.
And nobody needed to know why.
~~South Island, a long time ago~~
Sonic zipped through halls, bounding along merrily as he chased after a ball shaped robot. Ivo insisted he get exercise to maintain his powers, but the blue hedgehog ran fast enough to break his treadmills. So happy was he that Sonic face planted right into the big set of double doors at the end of the hall, feeling dizzy until they opened up.
"Sonic?" called a voice, the blue hedgehog looking up as his caretaker stepped over. "What were you doing, running around like that?"
"I was chasin' that ball, Ivo,'' replied Sonic, looking inside the room suddenly opened up to him. "Whoa, wat's this?"
Ivo didn't respond for a while, before eventually allowing the blue hedgehog inside. "This is where I work, Sonic. One day, I'm going to make a whole city."
A massive hologram appeared, taking Sonic's breath away. It was a massive city, with lots of buildings and a colossal tower right in the center. Words couldn't describe how impressed he was by it.
"What's this city's name?" he asked.
"Robotropolis, after myself, Ivo Robotnik." The hologram then changed to an emblem, a stylized version of the doctor's face. "It will be my ideal society, with no more ailments like poverty, hunger, illness, or free will."
The blue hedgehog was too young to know exactly what Ivo was talking about, but because his caretaker sounded happy, it made happy too. "I can't wait to see Robo… R-Rubbertopplepeas!" he cheered.
Ivo had to smile at this, bringing Sonic up so he could see the city better. "You'll get to see it soon," he assured, voice much lower as he added, "But not before I finish my plans for you, Sonic, Sonic, Sonic..."
Robotropolis, present day
"...Sonic? Sonic!"
The blue hedgehog jolted awake, feeling Cosmo shaking his shoulder. "We're here."
"Hmm? Oh, thanks, Cosmo." Sonic rolled off the cot he'd been sleeping on, springing to his feet. Memories were seeping into his dreams. "Alright, you two, let's get as many pieces from that tower as we can carry."
The Monsoon set down close to the edge of the metal city, Sonic stepping out first. Words couldn't describe how much he didn't want to be here.
"There's the tower," said Maisie, pointing out the fallen structure. "So, how do we do this?"
"Well, Tails didn't say how much of the stuff he needed, so I say we load the Monsoon 'til it's standing room only," suggested Sonic, his two companions agreeing.
As they moved about the site, taking chunks of the fallen tower and loading them into the Monsoon, the blue hedgehog couldn't help but glance around curiously. He knew that something was here the last time he came to the city, but he didn't remember what it was…
"Hey, Sonic." It was Maisie calling him this time. "Wanna give us a hand?"
"Y-Yeah, I was just trying to think of something."
"Honestly, you need to pay attention to your surroundings," teased the red rabbit, tripping over some debris and face planting on the metal ground.
Nearby, Cosmo giggled from the sight, the two electing to ignore it.
Deep inside one of Robotropolis' many underground vaults, red eyes flashed online. Another robot, this one different from other Badniks, detached itself from where it had been resting, detecting movement above. With speed rivaling Sonic, it disappeared from its chamber.
Back above, the three managed to get the Monsoon loaded up and were just about to leave when Maisie heard something from one of the buildings. Looking over to where it was, she was suddenly knocked onto her back, letting out a loud grunt. This caught the attention of the other two, Sonic's eyes going wide as he saw what it was.
Metal Sonic stood over Maisie, a foot keeping her head pressed firmly on the ground. Both hedgehog and Seedrian made note of the cerulean Chaos Emerald inserted in the robot's chest.
"Let her go, Metal!" snapped Sonic, his metallic copy turning to focus on him. "She's done nothing to you, and we're leaving. So just-"
Sonic took a step forward, Metal Sonic stopping him by putting more pressure on the red rabbit's head. As he wasn't focusing on Cosmo, she was able to slip out of his eyesight.
"What do you want?" the blue hedgehog demanded. "I know you're not a talker, but you can give me something." Metal Sonic simply pointed at his organic counterpart. "Me? You want me?"
Metal Sonic nodded, but didn't have the chance to do anything else before an axe blade slammed into his back, sending him forward and knocking the Chaos Emerald from his chest. Sonic acted quickly, grabbing the gem and carrying Maisie to safety, while Cosmo used her friend's weapon to brutally bludgeon the Elite Badnik. Sonic looked back and watched this, running back to grab her as well.
"If we leave now, you can kick his butt later," he said, closing the hatch on the Monsoon. "Tails, we've got the stuff. Get us out of here, now!"
The cargo plane roughly took off, Metal Sonic barely able to lift his head and watch it leave.
En route to Tails' Workshop
In the cockpit of the Monsoon, Maisie was checked over by her two companions. Besides a soreness to her skull and faint bruise where she had fallen down, she had no serious injuries.
"Thanks for the save," she told Cosmo, taking her halberd back.
"It was the least I could do," replied the Seedrian earnestly. The two stared at each other, their faces unconsciously inching closer, before the Seedrian shook her head. "Thank you, you're nice, but-"
"No, I understand. You're taken." Turning to where Sonic was sitting with the Chaos Emerald, Maisie found herself asking, "Is that another of those power-ups? What was it doing in that thing's chest?"
"Well, Chaos Emeralds are immense power sources, and keeping a robot like Metal dormant for so many years requires a lot of energy. Egghead probably plugged it into him before he ran off scared," explained Sonic.
"Right, so… do we need a third Chaos Emerald?"
"No, but we're better off keeping it instead of Metal."
This question came from Cosmo. "Why did Eggman create a robot copy of you, Sonic?"
The blue hedgehog smiled, but this one was more rueful. "He couldn't have the genuine article, I suppose."
Robotropolis
Metal Sonic tried to get to a standing position, but his limbs were too badly damaged and a lot of systems were going offline. Just as it seemed he may shut down for good, a shadow loomed overhead, and some sort of tractor beam picked up the metal replica and carried him into the strange craft.
Finding the stark white interior familiar, Metal Sonic turned his gaze as an especially familiar set of legs approached him. "I knew you were still functional!" he boomed, sounding almost pleased. "Don't worry, I'll have you back on your feet in no time." As he was turned onto his back by familiar geometric attendants, the visage of Dr. Eggman was almost… comforting. "Come on, you, we're going home."
Fun fact: Tails' favorite car is the Lotus Super Seven.