After accidentally losing her oxygen bottle while diving, Gi is rescued by a young man named Pontis and taken to a secret underwater city called Oceanus. Its leaders decide to detain her indefinitely to keep the city hidden from the outside world. However, the city is threatened by Sly Sludge's illegal toxic dumping.
This episode includes examples of the following tropes:
- Domed Hometown: A thick glass dome is the only way for the city of Oceanus and its inhabitants to not be crushed under tons of water.
- Gilded Cage: At first, Gi considers Oceanus a paradise, given that it's all about her element, but changes her mind upon hearing that she's not allowed to ever leave the city in order to keep it a secret.
- Group Hug: When they're finally reunited, Linka and the boys swarm over Gi, all five of them being glad to see each other.
- He Knows Too Much: Gi gets taken to Oceanus after getting saved by Pontis. Though they don't try to kill her, the grand council decide to imprison her in Oceanus forever in order to keep the existence of the city secret.
- Ship Tease: Between Gi and Pontis, due to them bonding over marine biology. While Gi can't stay in Oceanus, she does give Pontis a hint that they can meet on Saturday when she'll be scuba-diving – with a playful wink for good measure.
- Underwater City: Oceanus, hidden deep in the ocean and protected by a huge, thick glass dome.
