Chapter 840 is titled "Iron Mask".
In the laboratory of the Germa castle, Yonji reveals to Sanji that Germa 66's soldiers are all clones of past exceptional soldiers, developed using the joint research of Judge and Vegapunk. Ichiji and Niji then enter the laboratory, and Sanji attacks Niji for beating Cosette. Using Zeff as a threat, Niji retaliates and overpowers Sanji. Ichiji reminds Sanji of his position as their family's failure, leading Sanji to reminisce further about his childhood.
In Sanji's recollection of their early childhood, the Vinsmoke children were informed by their father that their bodies had been physically and intellectually enhanced using genetic manipulation, and that, through training, the siblings would develop the strength to lead Germa 66 to new glories. However, Sanji was continually outpaced by his siblings, leading them to bully him. Eventually, intent on erasing Sanji's existence from his plans, Judge held a state wake to "mourn" Sanji's accidental death; simultaneously, he had soldiers place an iron mask on Sanji and lock him in a dungeon, causing Sanji to break down and apologize piteously for being born weak.
Vinsmoke Yonji reveals to Sanji that the people kept in bottle-like tanks are the soldiers of Germa 66, and a shocked Sanji notes that they are all identical and wonders if they are even alive. Yonji tells Sanji that the Germa Kingdom has always been a kingdom of science, revealing that their father Vinsmoke Judge was hailed as a brilliant scientist before they were born. Judge was once part of a lawless research team, in partnership with Vegapunk, and the two discovered the Lineage Factor of living organisms, giving them the blueprints to life itself. Upon discovering this, the World Government decided it was too great a power for them to have; they arrested Vegapunk and drove the other researchers apart. Judge continued this research in secret, leading eventually to the creation of the clones that served Germa 66 as its foot soldiers. Yonji states that other nations not only fear Germa, but also envy their possession of an extremely loyal and easily replenishable military force (a 20 year old clone can be grown in five years).
Sanji tells Yonji to stop, stating that the revelations were making him nauseous, and he sits down and lights a cigarette. Vinsmoke Ichiji and Vinsmoke Niji then enter the room, saying that, when the family first learned that Sanji had been spending his recent life as a pirate, they had hoped it might have toughened him up, but all they had seen indicated that Sanji was as disappointing as ever. Upon seeing Niji, Sanji grows enraged at what he did to Cosette and kicks him in the face, but Ichiji reminds him that they can easily kill Zeff in response to any resistance by Sanji. Niji then moves behind Sanji extremely quickly and grabs him around the neck, taunting him over his powerlessness before kneeing him in the back. Niji's knee is electrified, and the attack causes Sanji to cough up blood as Niji punches him into the ground with an electrified fist. Niji tells Sanji that if he knew he cared about Cosette that much, he would have beaten her even more, and then arranged for a crippled Cosette to serve in Sanji's personal staff. Ichiji welcomes Sanji "home" and reminds him that he is at the bottom of their family's hierarchy.
As he listens to his brothers' taunts, Sanji recalls further parts of his childhood. As children, he and his siblings stood before Judge as their father informed them that they were his greatest scientific achievements; through his scientific breakthroughs, Judge had genetically enhanced their bodies to surpass normal human limitations. He instructs them that, if they train, they will be able to develop superhuman abilities and lead Germa 66 to achieve Judge's ambitions. As the training regime begins, however, Sanji quickly began lagging behind his siblings in their routines, which irritated them. Sanji faced off against Niji in swordsmanship training, but his wooden sword broke when it came into contact with Niji; Niji then easily beat him up with his sword, to the point where Germa 66 soldiers had to pull Niji off Sanji to prevent further damage. Later, a Germa 66 aide reported to Judge that Ichiji, Niji, and Yonji were all developing rapidly, both physically and intellectually, but Sanji's genetics had not changed since he was born, leaving him as a normal human being. An enraged Judge soon discovers Sanji in his room, having just cooked a plate of food for a rat. Judge proceeded to berate Sanji for serving others, stating that Sanji's mother had long passed and that there was nothing to be gained from following the legacy of the dead. Judge warned Sanji that, since he did not have his siblings' aptitude, he would have to train much harder to contribute to the Germa Kingdom. Judge then shattered a window, sending the rat and Sanji's dish falling toward the ground, causing Sanji to cry.
As their training continued, Sanji was once again beaten up by his brothers while Vinsmoke Reiju watched on and laughed; later, Reiju treated Sanji's wounds and confided to him that she only laughed at their brothers' actions so she would not be bullied herself. Judge and his aides continued observing no progress from Sanji, and Judge eventually decided to hold a state funeral to mourn Sanji's "death" in an accident. As they watched on, Sanji's brothers expressed a mixture of surprise and glee at the news of his death, while a nervous Reiju smiled at their conversation. In reality, Sanji had been detained inside the palace while other Germa 66 soldiers detained his face in an iron mask; he was then imprisoned in a dungeon, and began yelling for someone to speak with his father. One of the soldiers then revealed to Sanji that they had been acting under Judge's orders, as Judge wished to start over in a world where Sanji had never been born. Sanji began to cry in his cell, as he shouted for his father to let him out and apologized for being born weak.
- It is revealed that Judge and Vegapunk were formerly scientific research partners, until Vegapunk was arrested for discovering the blueprint of life.
- The soldiers in Germa 66 are cloned humans unaware of their status.
- Sanji is mistakenly drawn without the exploding wristlets on.
- Niji is confirmed to be the one who attacked Cosette in Chapter 839.
- Sanji and Niji have a brief confrontation which ends with Sanji being overpowered.
- More of the Vinsmoke Family's past is revealed:
- Judge genetically modified his children, but Sanji apparently failed to receive any enhancements.
- The Vinsmoke matriarch (and the children's mother) passed away some time during the children's childhoods.
- Due to Sanji's failures, Judge masked him and locked him in a dungeon before claiming to the kingdom that Sanji had died in an accident.
- Both a Devil Fruit Encyclopedia and a copy of Liar Noland are shown in Sanji's flashback.