Chapter 691 is titled "The King of the Land of Death".
Following Law's attack, the mountains on Punk Hazard have been sliced in half, allowing Shinokuni to enter the facility. Zoro, carrying Tashigi, meets up with Sanji, Nami, and the G-5 Marines, and is told to turn around because he is going the wrong direction.
In the SAD production room, Vergo has been defeated and cut into pieces by Law, but still tells the Shichibukai that he will fail because he does not know Doflamingo's past. Smoker and Law then leave, and Law tells Vergo that the room will soon be destroyed.
Meanwhile, Caesar realizes that it must have been Law that cut open the laboratory, and then tells his men to open the air vents and allow Shinokuni to come in. His subordinates hesitate, then, believing that is all an act to deceive their enemies, follow his orders. Caesar then merges with Shinokuni and kills some of his men as an example to Luffy. Luffy turns and runs down a hallway away from Caesar. Usopp, on the upper floor of Building B, asks Caesar's subordinates to let him use the control panel. His subordinates wonder why Luffy is running, and Usopp states that Luffy will not run. Luffy then turns around, enters Gear Third, and prepares to attack Caesar with both arms.
The chapter opens by showing the top half of the bisected building falling back down as some gas begins to leak in. The giant children are scared at first, but Sanji and Nami convince them to keep running. The G-5 Marines soon spot Zoro approaching carrying Tashigi while she demands he put her down. Upon seeing the Marines she realizes he has gone the wrong way. Sanji is furious that Zoro picked her up, while Nami tries to make him turn back.
In the SAD production room, Vergo has been completely dismembered by Law, and his body parts are attached to a railing. He calls Law's plan a messed up attempt at revenge, and taunts Law saying that he will regret it, and cannot replace Doflamingo with power alone. He then asks Smoker to agree with him, but Smoker remains silent, while Law cuts Vergo's head in half one last time, and states that Vergo does not need to worry about him but his own self since the room will soon be completely destroyed, before leaving.
Meanwhile, Caesar realizes his plan is falling apart, and becomes increasingly enraged. He orders his men in the secret room to open the air vents, letting Shinokuni flood the room, and when they argue that there are over a hundred men under Caesar's command left in the room that will be killed, he snaps at them, referring to them as guinea pigs and scum, and that since nobody cares he can keep killing and replacing them as many times as he wants. His remaining minions are shocked into silence by this, to the extent that the ones previously fighting Usopp outright ignore him while he steps back to hide.
Caesar's puzzled reaction to their shock convinces some of his men that he is bluffing, and did not actually intend to hurt them, and, believing it to be a complex strategy, open the vents as ordered. As the gas fills the room, Caesar sucks it into himself, and begins to grow huge, before taking on a more demonic form. Gloating about his new found might, he promptly murders all of his men remaining in the room as a show of strength, enraging Luffy. He then states that some peaceful countries have expressed an interest in his genocide weapons, and that he plans on supplying everyone, ultimately becoming the ruler of a war-torn "land of the dead". Luffy appears to flee, telling Momonosuke to protect Brownbeard.
In the secret room, Caesar's remaining forces are horrified by his casual murder of their allies, and realize he truly does not care about them. Usopp then interrupts, asking if he can use the control panel to help his friends and allies escape. Caesar's subordinates point out that Luffy has abandoned him, watching him run down the corridor on the monitors. Usopp counters that Luffy would find things easier if he did, but that was not in his nature. As he speaks, Luffy, still enraged, stops and turns around to face Caesar again. As Usopp continues to talk about Luffy's positive characteristics, his captain continues by blowing air into both his arms, expanding both of them to activate Gear Third. He then begins to charge back down the corridor towards Caesar, both his arms stretching out behind him. Usopp states that Caesar is the type of person Luffy hates the most, and that Luffy won't forgive him. The chapter ends as Luffy infuses Haki into both of his stretched arms, and jumps towards Caesar, entering a pose similar to that of his Gomu Gomu no Gigant Bazooka attack, while stating that he does not want to see Caesar's face anymore.
- This chapter's cover has the most characters of any cover page so far, showing almost all the named (and several unnamed) Marines that have appeared until now other than the full Straw Hat Pirates, Z, Ain and Binz, with a total of at least ninety four recognizable characters.
- There is also Donquixote Rosinante who is in-between Zoro and Nami with her Clima-Tact crossing out his face similar to the cancel sign that makes up the Donquihote Pirates' Jolly Roger.
- Law splits Vergo's body into many pieces and attaches them to the railing.
- Zoro and Tashigi catch up with the G-5 Marines.
- Chopper finishes his medical treatment of Mocha.
- Smoothie and Fen Bock, along with all of Caesar Clown's other subordinates, find out that they are actually being used as Caesar's guinea pigs.
- Caesar forces his men to open up the air vents in order to let Shinokuni in, which he soon fuses with to become stronger.
- According to Caesar, two rather peaceful countries already want his weapon.
- Luffy prepares to deliver a powerful attack to Caesar in Gear Third.
- Pandaman appears on the cover page between Franky's left arm and leg.
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