In 1915, Don Juan, Elizabeth, their dog Smile, and their pet mice Top Connors and Ronnie, are tasked by the U.S. Navy to explore the ocean depths in search for the wreck of the Titanic. A gang of sharks in prison attire find the descending bathysphere and summon their leader named Iceteeth. He orders them to cut the cable of the bathysphere that the humans and their pets are on and sink it, because he dislikes the craft’s yellow color. After the shark attack, the craft falls and wedges between some rocks, leaving the occupants unconscious. A giant octopus named Tentacolino finds the craft and tries to pull it out, recognizing the occupants on-board. Suddenly, a host of merpeople arrive to fill in for him, explaining that the surface is too far and the occupants will perish due to a loss of oxygen from the impact. They enter the craft and use spraycans to install bubbles on their heads so they don’t drown. As the merpeople travel back to their base, the group awaken and find themselves in Atlantis, where the merpeople live. Smile the dog is at first terrified by this, thinking he’s in “doggy hell”, but calms down shortly after.

The expeditioners undergo a procedure of drinking an elixir which enables them to breathe underwater before entering the city. Pingo, a living toy silver-fish who is one of the ambassadors for Atlantis' monarchy, reveals to them through a musical number in a room full of living toys, that they cannot go back to the surface, shocking the group. Later on in the main square, Smile falls in love with a white female Cocker spaniel and attempts to woo her, eventually delighting her with a rose planted on her hair. Meanwhile, the pet mice Ronnie and Top Connors are approached by a couple of rats and attend a secret meeting of a larger group of mice and rats, where they learn about a conspiracy to steal Atlantis’ elixir of life so as to gain immortality and strive for world domination. The group is led by a pompous but naive propagandist rat and is in cahoots with the sharks and their leader Iceteeth from earlier. Ronnie and Top Connors alert the others about the plot. The king of Atlantis decides to have the two mice spy for him on the rats’ plan, and substitute the elixir flask with a flask of ordinary water so as to sabotage the plan.

The sharks send a telegram to a boat on the surface, where a couple men are on their own expedition to find a perceived treasure in the Titanic. They are told to be on guard and support the sharks and rats on their mission. During a battle between the merpeople and sharks, a young toy soldier is promoted to be the new general of a 19th century-style artillery group after being at first mistaken for a little girl on account of a wig he wore. The merpeople use an amnesia laser to zap the ship on the surface, making the men forget what they were doing, so as to repel them. Meanwhile, the rats and mice steal the fake elixir and present it to their rat leader and a mouse elder, who is used as a guinea pig to see how the elixir works. After consuming one drip of the flask, the elder is tied to a couple of bricks and dropped into a deep well of water which almost drowns him, as a few of the mice jump in to save him, realizing that the elixir was illegitimate. Infuriated, the elder demands the rat leader and his fellow conspirators be committed to a mental hospital in the sewers and be kept there forever, despite his desperate pleas.

As a reward for their help defending Atlantis from the sharks and rats, the king assists the expeditioners in the Titanic's recovery, enlisting the assistance of Tentacolino and Pingo. The king then transports the Titanic and the expeditioners to the bay of a secret island, where Don Juan and Elizabeth will live happily ever after, while being able to return to Atlantis whenever they wish.

• Jane Alexander – Elizabeth
• Anna Mazzotti – Ronnie
• Francis Pardeilhan – Don Juan
• Gregory Snegoff – Smile

Original Italian cast edit

• Rodolfo Bianchi – King
• Fabio Boccanera – Don Juan
• Paolo Buglioni – Ice
• Stefano Crescentini – Top Connors
• Oliviero Dinelli – Tentacolino
• Luigi Ferraro – Mouse Pirate
• Christian Iansante – Baron von Tilt
• Beatrice Margiotti – Queen of Atlantis / Sea Amazon #2
• Stefano Mondini – Smile
• Fabrizio Vidale – Cutter

Tim Brayton of Alternate Ending gave the movie a 0.5/5, stating "Among the very top tier of so-bad-they're-good movies of the last 25 years."^[1]

• Tentacolino at IMDb