Chapter 931 is titled "O-Soba Mask".
Cover Page request: "Frogs tap dancing happily in a puddle that Nami made." - PN Noda Skywalker
Meanwhile, Robin tries snooping through Orochi Castle, but gets caught by the Orochi Oniwabanshu. On the coast of Kuri, Chopper, Momonosuke, Tama, and Kiku discover that Big Mom has washed ashore. However, she is unable to remember anything about herself.
Sanji pulls out his Raid Suit canister to disguise himself, and the Flower Capital citizens are surprised that he actually showed himself, thinking he will be killed. Sanji decides it is worth it to put his pride aside to save the citizens, and activates the Raid Suit, saying that wearing it does not make him part of Germa and he might as well try it out. The transformation brings Sanji up onto a roof, donning him in a black outfit with a mask over his face. Law realizes that this is the suit of a character known as Stealth Black, having read Sora, Warrior of the Sea growing up. Page One asks what Sanji's name is, and Sanji calls himself "O-Soba Mask" before using his jet-propelled boots to boost himself forward. Page One lunges at the roof, but misses Sanji and loses sight of him. Sanji then kicks Page One out of nowhere without being seen, and Law reveals that Stealth Black is a fictional member of Germa 66 who is able to become invisible at will. Sanji remembers wanting to turn invisible since he was a child, and notes how ironic it is that the Germa technology he hates is what made this dream come true.
After being sent crashing into a building, Page One gets up and transforms into his hybrid form, wondering who Sanji really is. He charges and swipes at Sanji with his arm, and although Sanji counters it with his foot, he is blown through several buildings by Page One's sheer strength. However, he only feels a little pain thanks to the Raid Suit's protection, and since Page One has lost sight of him, he thinks Sanji is dead and prepares to bring his body to Kyoshiro. Law then runs off with Usopp and Franky, pointing out that Sanji told them to leave without him and that they likely cannot stay in the Flower Capital much longer, and so he intends to head to Ebisu Town. Sanji then flies up and says that he will have Usopp and Franky look into modifying his suit afterwards, before he unleashes a surprise attack on Page One from above, creating even more damage that Law's group sees from afar.
In Orochi Castle, as Orochi continues partying the night away, Robin sneaks through a courtyard into an old room containing many file cabinets. Robin thinks that a Poneglyph is likely somewhere in this country and looks around for a secret entrance, when she is caught by the Orochi Oniwabanshu, a group of ninja in service to Orochi. The Oniwabanshu mock Robin for acting suspicious around them, and their captain Fukurokuju then enters the room. Fukurokuju gives Robin just one chance to explain herself and her actions.
Meanwhile, Tama, Chopper, Momonosuke, and Kiku head to Kuri Beach, and Tama is worried about Luffy, as her master had said that no one escapes from the Udon prison. They then notice a massive person lying down on the beach, and after getting closer, Chopper realizes in horror that it is Big Mom. Chopper quickly tries to get the rest of the group to be quiet and run away before she wakes up, but when he realizes that she likely came to Wano to pursue his crew, he quickly decides to kill her with his sword. However, the constant questions about Big Mom by the Wano natives cause Chopper to shout, resulting in Big Mom stirring. Chopper and Momonosuke quickly run away as Big Mom awakens, but Chopper is then stunned as Big Mom wonders who she is, having lost all of her memories.
- Sanji's raid suit is revealed.
- Franky, Usopp, and Law flee the Flower Capital and head for Ebisu Town.
- While sneaking around in Orochi's castle, Robin is found by the ninja troop Orochi Oniwabanshu.
- According to Tama, nobody has ever escaped from Udon Prison.
- Big Mom ends up at Kuri Beach and she has amnesia.