Context: adventure story, most likely (during) a Manichean type of conflict. Good guys versus bad guys. Single characters or groups, doesn’t matter for either side.
During one of the fights between the protagonists and the antagonists, one of the antagonists, a particularly antagonistic character is captured/taken in by the protagonists.
At first he tries to free himself/leave and go back to his own companions. He doesn’t succeed and is stuck with the protagonists for some time.
Over the course of said time he tries to destroy them or convert them to his point of view to get them to come to his side but he doesn’t seem to succeed.
Slowly, however, and without completely realizing it, he starts to turn to the protagonists side and to befriend them.
At one point in the story he even begins to help them towards their cause. Overtime he changes to their side.
A possible twist to that would be that one of the protagonists, at one point in the story (possibly around the moment the antagonist who has been captured starts becoming a protagonist), betrays the other protagonists and decides to join the antagonists’ side.
Why not make this happen during a great battle (meaning a major conflict in the story). The antagonist who has been captured finally starts helping the protagonists and as they are about to overcome (or even things out with) the other antagonists (who feel betrayed by the captured antagonist’s desertion) they are betrayed by one of them and that betraying protagonist leaves with the antagonists and joins their cause but not before badly wounding the captured antagonist.
This could be either about the point of view of the good side or the bad side: a dark/evil character becoming good (with all the struggle it implies), or a good character becoming evil. Or something else entirely.
Also, I say ‘he’ when talking about the character(s) but that doesn’t mean I mean them to be male (nor human), anything goes.
This may seem common (it uses some common tropes/plot devices) but the whole psychological aspect of the betrayal of the antagonist towards his companions and then the betrayal of the protagonist to join the other side could be very interesting to exploit.
Also, why not mix in a few love interests here and there to make things more complicated? Like between a protagonist and the antagonist they captured or between a protagonist and the other one who will betray them? The person the antagonist who starts to become good likes is killed (or gravely wounded) by the protagonist traitor?
Yep. There you go.
This is a short and simple one to begin with, but quite interesting and efficient nonetheless.
I hope you enjoy!