The episode starts with Connor, Amaya, and Greg wandering around the city. Suddenly, they see cars stacked up along with green goo. They decide to investigate.
They transform into their alter-egos and head to headquarters. On a camera, Romeo's lab can be seen by the river, but the cockpit is nowhere to be found. Instead, the cockpit is underwater with Romeo inside. They don't know what he's up to, but Owlette says that if anyone can defeat him, it's Gekko, the Master of the Deep. That being said, with the Gekko-Mobile, they head into the cave and Romeo is seen. He shows the trio the green stuff, which is power pondweed. He says that it grows by the crystal at HQ. With the pondweed, he's going to use a gadget to zap and trap what he wants, which will help him with his destiny to take over the world. He tries zapping the trio, but they get out of the way just in time and hide behind a rock. Catboy has a plan, but Gekko gets a different plan, so they go with the latter. Catboy and Owlette come out from behind the rock and distract Romeo. He locks the weapon on Owlette, but it suddenly starts floating. It turns out Gekko used his camouflage to snatch the weapon. He runs away with it, but Robot steps in and grabs the weapon, causing a tug-of-war. The invention is launched into the air, firing beams. One of the beams hits Owlette and Catboy, and both of them are trapped. Gekko tries to break them free, but his strength isn't enough. Catboy and Owlette, though, want Gekko to go after Romeo, so Gekko decides to do it. Romeo notices the Gekko-Mobile coming at him and zaps it. The vehicle is disabled, but Gekko remains calm and decides to go after him without it. However, he couldn't keep up with Romeo, and he's thrown into the pondweed. For a moment, he falls unconscious, having used his abilities to try and catch Romeo. Suddenly, he wakes up, and sees that he had absorbed the pondweed power, much to Romeo's dismay. With the power now gone from the pondweed, Gekko throws the cockpit onto the surface and swims back into the cave. He uses the pondweed power to free his friends. Just then, Romeo comes back with his gadget and fires it, but Gekko blocks the beam with his shields. Owlette uses her feathers, and they swirl around Romeo. Then, Catboy uses his stripes to take the gadget. Gekko breaks it with his strength and gives it back to Romeo. He leaves, followed by Robot. Just then, the Gekko-Mobile rises from the water with PJ Robot controlling it. They all say their signature victory phrase, ending the episode.
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Romeo shows to know about the crystal that powers the PJ Masks in this episode.
This is the second episode to have an underwater mission.