The story opens with Brother remembering the scarlet ibis in the "bleeding tree" (1). This frames the story as a memory. We know that the events Brother describes happened in the past, though we don't know exactly how far back in the past. We also know that the memory is about Doodle, though we don't know what Doodle and the ibis have to do with each other, yet.
As we discuss in "Writing Style" a story with a frame is considered a story within a story. The fact that Brother is remembering something is one story. What he is remembering is another story. Here we are focusing on the plot structure of that second story, the story in Brother's memory, which begins when Doodle is born, when Brother is six. Baby Doodle is born with some physical problems, though we aren't, initially, given any details on these problems. Most of the family thinks Doodle won't survive. Daddy, in fact, goes so far as to have a coffin built for the baby.
It sounds a little mean to say that Doodle's living creates the conflict in the story Brother is remembering. In the 1900s, when the memory is set, even doctors didn't know how to deal with a child with a child like Doodle who, we learn, has a heart problem. Still, the doctor seems to be behind much of the conflict in the story. By leading the family to believe that Doodle a) won't live b) might not have below normal brain function, and c) that he can never walk or live a "normal" life, the doctor makes Doodle seem extremely abnormal.
When Mama tells six-year-old Brother that Doodle will never be able to run and play with him, and that Doodle "might not" be "'all there'" Brother decides to kill him. He says, "It was bad enough having an invalid brother, but having one who was […] not all there was unbearable […]" (2.3). The gap between Brother's idea of what a brother should be and who Doodle is lies at the heart of the conflict.
When Doodle smiles at Brother he realizes that the adults around him are wrong – Doodle is intelligent. When Doodle learns to crawl and talk, the things the doctor said he would never be able to do, Brother begins to see Doodle as his own special project. He wants to close the gap between what he wants Doodle to be and what Doodle is. He also wants to protect Doodle from people who might hurt him because they don't understand him.
In Brother's memory, teaching Doodle to walk is an entirely selfish act, done to turn Doodle into someone he can be proud of. This is the story's big complication, the complication that drives all the events in the story. It is also what creates the complication in Brother's mind. He feels like he's doing something good, but for the wrong reasons. If he were simply trying to help Doodle, for Doodle's sake, things would be far less complicated for him.
For the family, seeing Doodle walk on his own is the story's big emotional climax. What do we mean by that? What exactly is a "climax"? Well, a climax is any big physical and/or emotional moment that the story has been building toward from the beginning of the story. Most stories have several such moments, so to locate the climax we have to look for the biggest such moment – not including the moments found near the end. Usually, the climax happens near the middle of the story. What happens in the climax usually sets the rest of the story in motion, and prepares us in some way for an even bigger physical and/or emotional event at the end.
When Doodle walks in front of his family for the first time he and his family, and (hopefully) the readers experience an outpouring of emotion. It works because we can all imagine how it would feel if someone in our family who we thought would never walk is suddenly walking.
For Brother the climax released all sorts of conflicting emotions. He's ecstatic at first, as shown by his dancing around with Aunt Nicey. When his family learns that he taught Doodle to walk and begin hugging him, he feels emotional pain and he cries. This emotional pain is a result of extreme pride and extreme shame. In fact, for Brother there seems to be little difference between pride and shame. He says that because he was pride's "slave" he was "ashamed" of Doodle (3.18). He wanted Doodle to walk so he could be proud of him, no longer ashamed.
So now the story has reached such great heights, it has to give us something special to keep us reading. Suspense is the tension between what we want to happen to the characters and what we fear might happen to them. The more we care about the characters, and the more we can relate to their experiences, the more suspense we feel.
In the case of "The Scarlet Ibis" this is pretty simple. We want Doodle to be OK and we fear that Brother's intensive training program might be too much. When Doodle shows signs of exhaustion we might wonder if he'll make it to the first day of school. This fear for Doodle is intensified by all the mentions of death and dying in the story – from the pine coffin, to the dead ibis.
Dénouement is a French word, that means, "the final unravelling of the complications of a plot in a drama, novel, etc.; the catastrophe" (Oxford English Dictionary). The catastrophe is the thing that happens that makes the end turn out the way it does.
In "The Scarlet Ibis," this stage probably begins when Brother and Doodle go to Horsehead Landing after the ibis incident. After Doodle overexerts himself rowing, Brother realizes that there is no way Doodle is going to be the way he wants him to before the first day of school. He forgets that Doodle really is fragile, and sensitive. This might have something to do with the fact that Brother is only thirteen.
When Brother runs away from Doodle, abandoning him, something we (and Brother) already know Doodle is deeply afraid of, our fears for Doodle intensify. We also know that if Doodle gets hurt, Brother will feel responsible. When Brother gets over his surge of irritation and turns around to go back to Doodle, all we can do is hold our breath, hope, and read on…
When Brother gets back to Doodle, Doodle is dead. Having bled from the mouth, his neck is covered in blood, and he reminds Brother of the ibis Doodle buried just a few hours before. For the first time, Brother's real feelings toward Doodle come out. By crying, screaming Doodle's name, and covering Doodle with his body, Brother shows the reader that he loves Doodle, and that all along he'd been trying to protect him from an outside world that doesn't tolerate people who are different. This is a complicated moment on its own, made even more complicated by the fact that Brother connects the dead ibis with Doodle. So, you might want to check out "What's Up With the Title?"