One Piece Strong World Part 1

The attack was a handful of marine fleets floating above a marine base, with its giant ship/island floating by above the base's headquarters as if mocking them. Then, as the ship passed, he said "This is a warning,". Then all the marine ships crashed to the ground unceremoniously.
Garp and Sengoku looked over the attack and Garp mentioned how he only knows only one person who could perform such an attack, which was the man defeated by Gol D. Roger and was then arrested; he was a legendary pirate, the sole survivor from the days of Roger and he went into hiding for 20 years after his amazing escape from Impel Down. He was the man who freed himself of his shackles by cutting his legs off.

That man was, 

Shiki, the Golden Lion.

Luffy reached a clearing after going through a forest, alone and worn out, and instead of enjoying the mesmerizing few of the multiple floating islands in front of him, he told himself that he needs to find the rest of his crew, and then jumped down.

In the next island, he was chased by a giant crocodile and then a giant octopus which defeated the crocodile to get Luffy. Luffy ran, and suddenly, a praying mantis appeared. Luffy watched the battle of those giant fighting creatures with interest, and after another giant beast appeared to destroy the praying mantis, that beast went after Luffy next. Feeling challenged, Luffy fought back against the beast and he had to use his Gear Third to defeat it.

In an autumn setting with beautiful trees incorporated into the buildings, Brook noticed a sound and turned around, whereas Franky and Robin walked on, ignoring the warning. When tons of small ants went to charge towards them, they didn't attack Brook and went straight towards the other two. Franky, having ran out of fuel was not able to fight back, but thankfully, a giant creature, seemingly a combination of an eel and shark went for them as well and gave them time to run away as well as a distraction for the ants. The sea creature surfaced again, only to be attacked by the whole ant army which emptied the whole creature of its flesh, leaving only the bones to float on the water.

Franky told Brook that the reason the ants do not attack him is because Brook is all bones and no flesh. Brook then attacks with Three-Verse Humming Arrow Notch Slash, feeling offended. 

"I am all bones, but there is something that I want to do. Lose weight in specific parts of the body."

Franky and Robin note that the creatures in the island are strong and fight to survive. 

Nami was in a garden, with glass as its walls and a huge swimming pool. As she takes a lap, she notices the drainage at the bottom of the pool. As she was drying herself off, her captives make a showy entrance with some dancing and music. They were Shiki, his bodyguard (gorilla) and a Dr Indigo (clown). Nami gives them the cold shoulder, reminiscing how she got captured just a week ago.

FLASHBACK BEGINS

She read from a newspaper that towns are getting destroyed every day in East Blue aloud to te whole crew. Luffy, Nami, Usopp and Zoro are from East blue whereas Sanji was born in North Blue and raised in East Blue. As Luffy read the newspaper, something suddenly appeared overhead and when he looked up, it was an island with paddles on its sides, a flying ship island. Nami noticed the change in weather; a cyclone was coming straight ahead. The ship above sent a Tone Dial down, proving that they have been to Sky Island, and Nami warns them, "A cyclone is coming! Turn your ship 90 degrees to the left!"

Shiki, being the cruel man he was, closed the main doors of his ship, refusing the entrance of his men outside, and shot down the navigator (who gave the wrong weather prediction) as a warning to the others. Interested in the excellent navigator, he floats down to the Sunny. He then told the crew that he ate the Float Float Devil Fruit, allowing whatever he touch to be able to levitate with his own will. Unfortunately, his power does not work on living things like humans and animals other than himself. 

Reaction to not being able to float.

Luffy rejected his invite to Shiki's hideout as a token and told him that they were heading to East Blue. Usopp reminded him of their hard-earned adventure and Luffy then said that they should protect their hometown.

Shiki offered to bring them to East Blue as gratitude but lead them towards multiple floating islands. "This is Merveille! A perfect island for adventurers like yourself! Enjoy it to your heart's content!"
He then grabbed Nami and escaped, not letting the Straw Hats a chance to get back at him as he made Sunny sink and levitate to attack the crew. This attack caused all the Straw Hats to be separated.

FLASHBACK ENDS

Dr Indigo brought a caged bird, claiming to have a new evolution but it escaped and electrocuted the dancing trio. Frustrated, Shiki threw the electricity-discharging-bird towards Nami. Nami stood in front of the bird, protecting it. Shiki explained to her the meaning of the 'evolution' they were speaking of.

The animals on the islands evolve strangely, influenced by a plant called 'IQ' which affects the animals' brains to adapt to the environment. Seizing this opportunity, Shiki took control of all IQ plants on the island. Dr Indigo managed to create a new drug called 'SIQ' from the IQ plant after 20 years. It evolves the animals to be violent.When Nami asked why he does so, Shiki said that he will only tell his crew, and are only willing to do favours for his dear crew.

Nami looked outside, a snowy terrain, and figured that the reason why they haven't rescued her is probably due to the animals.