Ep 18. Submersion

Sheppard and his team investigate an Ancient structure that Rodney believes to hold the secrets of eternal energy. However, when they enter the structure, they find themselves being submerged in water and trapped. They quickly discover that the structure is actually a research facility built by the Ancients to study the underwater civilization of the planet. As they explore the facility, they realize that they are not alone and that the underwater civilization may still be in contact with the Ancients.

Meanwhile, back at Atlantis, Sam and Zelenka are working on a project to create a new power source for the city. They are interrupted when a Wraith dart suddenly appears in the city and begins attacking. With no way to defend themselves, the team is forced to evacuate to a hidden location.

As the team in the research facility struggles to find a way out, they encounter a group of underwater creatures that are unlike anything they have ever seen before. They soon learn that these creatures are not only intelligent, but also have the ability to communicate with them telepathically. The team soon realizes that they may be able to use this telepathic ability to help them escape the facility and get back to Atlantis.

As Sam and Zelenka desperately try to come up with a plan to defeat the Wraith attackers, they receive an unexpected message from Sheppard. Using the telepathic abilities of the underwater creatures, Sheppard is able to communicate with them and relay a message to Atlantis. With this new information, Sam and Zelenka are able to create a plan to take down the Wraith.

As the team in the research facility continues to work with the underwater creatures, they discover that the Ancients had created an entire city beneath the water's surface. They soon realize that this city may hold the key to their escape. With the help of the creatures, they embark on a dangerous mission to find a way out and reach the underwater city.

In the end, both teams are able to work together to defeat their respective enemies and find a way to return to Atlantis. However, they are left with more questions than answers as they realize that the underwater civilization is much more advanced than they ever imagined and may hold the key to many of the secrets of the Ancients.