Kwazii and Dashi become entangled in a strange deep-sea creature - a siphonophore - and when a geyser blasts it toward the surface, the Octonauts must stop its ascent before it fatally bursts.
Water Bears: The Octonauts venture into a dangerous red-hot lava tube to rescue a teeny tiny water bear, who doesn't seem to mind the heat at all! Barracudas: Shellington and the Vegimals mission to replant a mangrove forest is put in peril by a school of toothy barracudas.
Humpback Whale: When a sardine swallows the key to a treasure chest, a humpback whale helps Kwazii track it down. Platypus: While rescuing river creatures after a storm, the Octonauts find a strange egg and come under attack by stinging platypuses.
As Tweak tests the Gup-X deep down in the Midnight Zone, a yeti crab cuts a vital electrical wire in the ship and cuts off all power, endangering itself and the Octonauts.
Red Rock Crabs: When Shellington and the crew try to gather some skittish crabs who have drifted away from home, they get help from some old friends. Sea Pigs: The Octonauts must stop a huge herd of sea pigs before they march blindly into the Mariana Trench - the deepest point on earth!
Examining a sick sea sponge, Peso is surprised to discover that all sorts of creatures live inside it, treating it like a hotel, a behaviour known as commensalism.
Mudskippers: The Octonauts attempt to keep the peace amongst a trio of mudskippers whose home has been flooded. Immortal Jellyfish: Shellington and the Octonauts try to track an unusual creature: a jellyfish that can transform from old to young repeatedly - and live forever!
Urchin Invasion: The Octonauts head to the kelp forest to visit Shellington's sister, Pearl, and her newborn son Periwinkle - only to find the forest endangered by an infestation of kelp-eating urchins! Lionsmane Jelly: A lionsmane jellyfish gets its long and stingy tentacles tangled in the bubble engine, unintentionally flooding the Octopod and causing havoc.
Pelicans: When a floating garbage patch endangers a flock of pelicans and other sea creatures, the Octonauts and the birds work together to clean it up. Hammerhead Sharks: The Octonauts study a group of baby hammerhead sharks, as they hunt on their own for the first time and poke their noses into trouble.
Sea Sponge: A sea sponge that houses other sea creatures in its pores suffers from a mysterious illness and Peso must figure out the cause. Loggerhead Sea Turtle: When an underwater tremor separates the Gup-X and its crew, Barnacles seeks guidance from a mysterious sea turtle.
When the Octopod goes in for repairs the Octonauts take a vacation, and Peso accompanies Barnacles to help his sister and her new polar cubs learn to navigate the harsh conditions of the frozen arctic.
Tweak has designed a brand-new Deep Sea Station to study the deepest, darkest reaches of the ocean, but the Octonauts face multiple challenges: from getting it down there, to the extreme environment and strange, prehistoric creatures that greet them once they arrive.
