"One Piece" chapter 840 focused on Sanji, and it is expected that chapter 841 will continue the story on his falling away with his family.
Previously, Sanji found head chef Cosette badly hurt. His brother, Yonji, told him where to find Niji, his other brother who tried to hurt Cosette before. He was then taken to the basement where he was shown the secret of Germa 66.
In Chapter 840, he finally learns that what he is seeing are the kingdom's soldiers that his father manufactured. He also learns that their old man is a scientist who used to worked with the famous Vegapunk even before the siblings were born.
After Vegapunk was arrested by the World Government, the Vinsmoke patriarch continued experiments on genetics and succeeded in making soldier clones that are loyal, strong, and dispensable. They can be manufactured in five years time, having the body, mind, and experience of a grown soldier. This angers Sanji, and a fight breaks out. He is no match for his brother, however.
A flashback then shows that the Vinsmoke kids were their father's "greatest creations," but Sanji ended up a "failure." He was not able to become as strong as his brothers and sister. He also remained kind-hearted, and it was hinted that he inherited this trait from his late mother. Because of his "normal" abilities, inferior to the others, the family declared him dead, but he was actually locked up.
The chapter ends with little Sanji yelling through his cell bars, asking his father to let him out and apologizing for having been born a "weakling."
In "One Piece" chapter 841, if the focus does not shift back to what's happening with Luffy in the forest or with Chopper inside mirror, then it is likely to continue with the story of Sanji's childhood. It might reveal how he got out of his prison cell and how he met Zeff.
Also, given that the Vinsmoke patriarch is a genetics scientist, it can be presumed that the Vinsmoke siblings are clones or somehow were genetically enhanced. Sanji, in his despair and anger, might finally discover how to use his innate and enhanced abilities in the same way that his siblings do, although this is mere speculation. If this happens, then he and Luffy might be able to face their enemies and stop the wedding.
