On the signage throughout Mysterious Island, Nemo himself outlines the mission of the attraction:
The sea has many secrets and much to teach us. I have devised a fleet of submarine boats to expedite my exploration of the sea's bounty.
One such sub-pod can been seen hanging from the chains of 20K's spiraling outdoor queue.
Beneath the surface, the entrance to the sub-pod bay winds past Captain Nemo's private offices and the diving suit staging area. Though evidently savvy to the Japanese language, a self-portrait in his office indicates that this is none other than the original Nemo from the film, portrayed by James Mason.
The line eventually brings you to the sub-pod bay, where you load into your boat and are dropped into the briny depths. At this point, Tokyo's 20K resembles those that have come before. Aboard your craft you visit the underwater farms, ship's graveyard and have a fateful meeting with a giant squid. One unique feature of the attraction is that searchlights periodically turn on that can be controlled by the joystick in front of your seat. With them you can focus your gaze on the funny and frightening fish of the various settings. After electrocuting your way out of the squid's grasp, you once again sink to depths beyond which man has not travelled.
Sure enough, the remains of Atlantis are to be found there. Here we finally complete the mystery of the Greek ruins flanking Mount Prometheus and the Fortress Explorations painting depicting the city's destruction. Then this 20K takes a turn into the weird, for something else lives in Atlantis now. Training your searchlight on the frescoes, you see the story of Atlantis' abandonment and occupation by a new species that may, in fact, be from beyond the stars.
Narrowly escaping a bizarre threat, your sub-pod returns to the dock and you make your way to exit. Waiting for you there is a friendly word from Captain Nemo's corporate sponsor:
20,000 Leagues is a dry-for-wet attraction much along the same lines as Peter Pan's Flight. The submarine pod suspends from a wire monorail and the water effects are all self-contained within the vehicle's bubble windows. The effect, however, is quite convincing. It also bears another, more subtle similarity, in that 20K is more an heir to the Fantasyland darkride in its mechanics than to previous 20K rides. As the pod swims through blacklit sets, your searchlight scopes out fluorescent fish sculpted in a cartoonish, often comedic style. For example, in the ship's graveyard, Venus statues are given back their arms and heads by the careful positioning of a comic eel. Several fish find themselves adorned with pirate hats. The... somethings... that now occupy Atlantis also have a cartoon style that almost seem to transpose 20K onto the medium of animation.
The associated Nautilus Gifts in Mysterious Island is sadly devoid of official 20,000 Leagues souvenirs, save for a Japanese copy of the DVD. The ride itself has purely Japanese narration to boot, but Disney remedied both of these problems in one fell swoop by publishing an English "story paper" for the attraction. "Story papers" are free slips of good stock paper printed with English text and excellent graphics that tell the premise of the ride for the English-speaking portion of the audience. Most attractions in Tokyo Disneysea have a story paper that is readily available for the asking (usually at the ride loading area) and they make fantastic mementos that really ought to be common at every park. The following is the story paper for 20,000 Leagues Under the Sea (click to enlarge):