The gang ventures into a red-hot lava tube to rescue a tiny water bear that does not seem to mind the heat at all.

Shellington and the Octonauts try to track an unusual creature: a jellyfish that can transform from old to young repeatedly - and live forever!

Kwazii and Dashi become entangled in a strange deep-sea creature - a siphonophore.

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!

When Kwazii gets lost at sea aboard the tiny octo-ski, he meets a flotilla of even tinier sea skaters - the only insects who live on the ocean.

Peso sets out to save the day when a cone snail invades the octopod and picks off the crew one by one with its dangerous harpoons.

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.

Shellington and the Vegimals' mission to replant a mangrove forest is put in peril by a school of toothy barracudas.

When Peso's patient, a remora fish, mysteriously disappears, the Octonauts enlist a unique undersea detective - a harbour seal.

A lion's mane jellyfish gets its long and stingy tentacles tangled in the bubble engine, unintentionally flooding the Octopod and causing havoc.

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.

After an undersea tremor separates the Gup-X and its crew, Barnacles seeks guidance from a mysterious sea turtle.

When a sardine swallows the key to a treasure chest, a humpback whale helps Kwazii track it down.

While rescuing river creatures after a storm, the Octonauts find a strange egg and come under attack by stinging platypuses.

The Octonauts attempt to keep the peace amongst a trio of mudskippers whose home has been flooded.

A sea sponge that houses other sea creatures in its pores suffers from a mysterious illness and Peso must figure out the cause.

The Octonauts meet some hammerheaded sharks, however, they can't sleep too well because Shellington and Professor Inkling have been arguing about who is smarter that they are no longer friends.

When an underwater storm destroys a small reef, Tweak and her crew try to convert the Gup-F into an artificial equivalent.

When Shellington and the crew try to gather some skittish crabs who have drifted away from home, they get help from some old friends.

The Octonauts must stop a huge herd of sea pigs before they march blindly into the Mariana Trench - where there will be no access to food. But things get chaotic when Tunip goes missing!