One possible plot could be that Spider - Man uses his agility and spider - sense to detect an enemy that the Avengers have overlooked. For example, he might notice some hidden Chitauri technology that is about to activate and cause chaos. So he swings in, warns the Avengers, and then together they figure out how to disable it.
Spider - Man's web - slinging ability can be really useful for the Avengers. He could create webs to trap enemies, which would be a great addition to the Avengers' combat tactics. For example, when fighting against a group of fast - moving enemies like Ultron's drones, his webs can slow them down, giving the other Avengers like Captain America or Black Widow an easier time to take them out.
Captain America could be his best partner. Cap has great leadership skills and a strong moral compass. He could teach Spider - Man a lot about being a hero in terms of responsibility and making the right decisions. Also, Spider - Man's acrobatic fighting style would complement Cap's more straightforward combat approach.
Well, maybe he's got some secret mission that he doesn't want the Avengers to interfere with. You know, like protecting a particular person or some crucial information that he thinks the Avengers might mess up if they get involved.
Another possibility is that the Avengers are being manipulated by a villain. The bad guy could be using false information to make the Avengers believe that Spider - Man is a danger. So they go after him without really knowing the truth.
Spider - Man would likely be confused at first. He respects the Avengers and can't understand why they are suddenly after him. He might try to talk to them, to explain that he's innocent. For example, he could swing up to Captain America and start pleading his case.
Spider - Man would probably be shocked at first. He looks up to the Avengers and being kidnapped by them would be the last thing he expects. He'd likely try to talk his way out, using his quick wit and charm. He might start by asking why they're doing this and try to convince them that they've made a mistake.
Spider - Man would be very confused. He would feel betrayed because he thought the Avengers were heroes he could trust. He could start by asking them why they are doing this. If they don't give him a satisfactory answer, he might try to escape in a sneaky way, using his spider - like abilities to find a weakness in their capture setup while still trying to reason with them.
Spider - Man would probably be defensive at first. He's used to working alone most of the time, and being interrogated by the Avengers would seem unfair to him. He might crack some jokes to ease the tension, but also firmly state that he's a hero too.
Yes, there might be such fanfiction out there in the vast world of fan - created content. However, this kind of plot is quite dark and not in line with the typical heroic and collaborative image of the Avengers and Spider - Man in the mainstream comics and movies.