Izumi decided to leave Ed and Al on Yock Island, an uninhabited island, the same place she left them during their alchemy-free training, hoping they would take the time to learn something. Winry can only watch as she, Izumi and Sig take the ship back to the mainland. As they go, both Ed and Al realize that they are not the same as before, as both of them can no longer swim.
They both remembered the day Izumi and Mason had left them there for a monthly evaluation test with the same rules to see if they were worthy of her training, and she left with the quote “One is all, all is one”, pointing out that they need to know the meaning to pass the test. Back in the present, Ed remembers how they had to work hard to understand it compared to the present, unaware that they were being chased by a mysterious figure. They then arrive at the place that used to be their bed, which they wondered what Izumi meant the first night. That night, the two also had to deal with a mysterious masked man who overpowered the boys and forced them to run and hide.
Back in the present, Ed notices some foxes as the mysterious person continues to observe them. As the current Elrics catch some fish with Al wading through shallow water without damaging his Blood Rune, they recall catching a bunny during their training days. Due to their reluctance to kill him, he was possessed by a fox, who then injured Ed’s original right hand when he confronted her. When Ed and Al finally find the fox’s nest and watch the fox give the rabbit to her young, they try fishing instead. After a few days, they caught some fish, but a masked man scared them off.
In the present, when Al tends to cook fish, a mysterious person observes him. Al accuses Ed of eating the fish when the fire is destroyed with just a handprint. After verifying with Al that he didn’t do it, the two go in search of this mysterious person.
In the past, the Elrics had to develop weapons and use other tools and avoid the masked man, but they had no luck and almost no food because the mushrooms on the island they found were inedible. Frustrated with their troubles, the past Elric brothers continue their search, and Al mistakenly believes the stranger to be Ed shortly before the real Ed returns.
During an encounter with the masked man in the past, once Ed saw him eating a dead cicada, he realized the importance of survival and was able to convince the masked man to let Al go and leave. After discussing this with Al, Ed now recognized how small his existence was, and if he died, his body would remain and eventually degrade as part of nature, part of the greater current. Ed realized it was something Izumi was trying to teach him because the flow of nature and alchemy worked together, which was the answer to her question. The two then worked together to find the right mushrooms and hunt for food, including killing a rabbit without hesitation, for the remaining days of training before they passed the test and continued training.
In the present, Ed realizes that the point is to teach them the concept again, due to their mother’s transmutation, as well as their current mission. After talking it over with the present Izumi, who has returned to the island with Mason, Ed decides to move forward. As Izumi notices the presence of another person, the Elrics believe it could be the same masked man from the past. After Mason reveals that he was the person because Izumi secretly sent him to watch over the boy, the stranger who is the boy comes out of the bushes.