One possible plot is that Sonic is exploring a new area. He comes across a small workshop where Tails is building his first plane. Sonic is intrigued by the little fox and his inventions. Tails is initially startled by Sonic's sudden appearance but then gets excited to meet his idol. They start chatting about their adventures, with Sonic sharing his tales of running fast and Tails talking about his love for inventing things.
In a fanfiction where Tails meets Sonic for the first time, Tails might be in awe. Sonic's speed and heroic reputation could make Tails instantly idolize him. Maybe Sonic saves Tails from a dangerous situation, like a group of badniks. Tails, being the smart fox he is, could then offer to help Sonic with his gadgets, starting a great friendship.
In this fanfiction, perhaps Sonic Underground is facing a threat that they can't handle alone. Tails, being a friend of Sonic from another part of their world, comes to their aid. They discover that an ancient evil has been awakened. The story could involve Tails using his inventions to analyze the ancient evil's weaknesses while Sonic and his siblings use their unique abilities to keep the evil at bay. As they progress, they form a strong bond and learn from each other's ways of doing things.
They might go on an adventure. Maybe they'll discover a hidden treasure or face a new enemy together.
In the fanfiction, when Sonic meets Tails in the snow, they could also just have a friendly chat. They might talk about their past adventures, like the time they saved the world from Dr. Eggman. Tails could share some new inventions he's been working on, and Sonic would be excited to hear about them. They might also plan their next adventure while enjoying the snowy scenery.
Their interaction would likely be positive. Sonic, being the friendly hero, would probably greet Tails with a big smile. Tails would be shy at first but soon open up due to Sonic's warm attitude.
Well, it could be that Sonic is dashing through the forest on one of his adventures when he suddenly hears a strange noise. He follows it and discovers Tails, who is trying to fix his broken plane. Sonic is immediately intrigued by Tails' two tails and his mechanical skills.
One possible plot could be that Sonic yells at Tails because Tails made a risky decision during a mission that almost put them both in danger. For example, Tails might have tried to take on a powerful enemy alone without waiting for Sonic's backup.
Well, Sonic might be a bit startled at first. Amy is known for her boldness, especially when it comes to Sonic. She could immediately start blushing and stuttering while trying to talk to him. Sonic, being the free - spirited hedgehog he is, might be a bit confused by her sudden appearance and enthusiasm. He'd probably be polite but also keep his distance, not fully understanding her feelings yet.
When they meet for the first time in fanfiction, Sonic could be running from Dr. Eggman and accidentally runs into Tails' hiding place. Tails is scared at first, but Sonic quickly reassures him. They start talking, and Sonic is amazed by Tails' intelligence and his ability to build things. Tails is inspired by Sonic's courage and they become friends right away, sharing their goals of stopping Dr. Eggman.
It could be a new villain created just for this fanfiction. Maybe a powerful wizard who wants to use the combined powers of Sonic Underground and Tails for his own dark magic. He sees their unique abilities as a way to gain ultimate power and they have to fight against his spells and minions.