When reaching Area 4, fall down the large pit, behind Agent 3 is more platform. That is where the kettle is to the 18th mission.
Two Spreaders and three Twintacle Octotroopers can be found, two of the Octotroopers are riding on the center of the Spreaders' axles while the other is positioned on the end of the furthest one. Splat the Twintacle Octotroopers as you head for the launchpad at the end to jump towards the next checkpoint.
The first part of this area is getting across a series of platforms. You are now faced with four Octocopters and three Spreaders. Taking out the Octocopters will make getting across the area a lot easier. But ride one of the ends of the longest Spreader that will take you to a far-off platform containing the mission's Sunken Scroll. Once you collect it, use the launchpad to return to the main route. When you do ink the steep slope with a short Spreader at the right area to easily get to the top and splat the Twintacle Octotrooper before getting to the launchpad towards the next checkpoint.
There is now a large one-bar Spreader inking a large platform with a lone Twintacle Octotrooper. Take it out and drop down to a platform below and cross the tri-bar Spreader to reach the next checkpoint.
When you climb up a narrow wall, there is a four-bar Spreader with two Twintacle Octotroopers positioned on two of its ends. One of them has the key to the vault of the far platform. Splat both of them and grab the Key and open the Vault containing the Launchpad leading to the final checkpoint.
Now you are positioned on a large platform with four one-bar Spreaders and a Vault in its center. You are then ambushed by an Elite Octoling. Take it out, which prompts another Elite Octoling to attack. Splatting it will reveal the Key to the Vault. Opening it will reveal a launchpad sending you to the Zapfish.