Echoes

"Echoes" is the twelfth episode of the third season of Stargate Atlantis.

Plot
Dr. Zelenka Ronon and Sheppard are in a Puddle Jumper returning from a mission on the continent to document and hunt local fauna. As they return, Zelenka notices that Atlantis does not appear on the censors properly. After adjusting their direction by forty degrees they are on the proper course towards Atlantis again.

Teyla Emmagan begins to see repeat visions of Ancients on Atlantis. First she is assumed to be under stress, but after a while other people on board begin to see the same visions.

Meanwhile, McKay and Sheppard see a sea creature which is named "Sam" (after Samantha Carter) by McKay, similar to that which led Sheppard and Zelenka to rescue McKay in "Grace Under Pressure" and decides to take a Puddle Jumper to investigate underwater. During a close encounter with the sea creatures, Sheppard and McKay develop intense headaches and McKay falls unconscious.

In the infirmary, Zelenka explains that the sea creatures emit a low frequency pulse and an electromagnetic field which, in combination, are very dangerous to humans. Very soon, most of the city is affected by this. Many victims are transported to the Daedalus, now orbitting the planet, but there are too many in the city to all be removed.

After recovering, McKay locates an Ancient bio-lab dedicated to studying the native marine life and manages to translate what the whales are saying. The Atlantean whales are warning that the sun is about to send out a gigantic coronal mass ejection (similar to a solar flare) straight at the Lantean planet and although the shield will protect Atlantis, the radiation wave will kill all plant and animal life on the planet and render the atmosphere unbreatheable. The whales are congregating underneath Atlantis for protection.

The visions that people have been seeing are a racial memory that the whales have of this catastrophe, which happens every 15,000 years. An Ancient ship was near to the sun when the ejection began and most of its crew were killed immediately by the radiation. The pilot managed to jump to hyperspace and overtake the radiation wave before it reached Atlantis and warn the people there.

Now knowing what is happening, the team consult the Ancient database and discover that the last time this happened the Ancients managed to temporarily expand the cities' shield to encompass a large percentage of the planets surface. Unfortunately, with only one functioning ZPM, McKay is only able to expand the shield to a radius of a few kilometers.

In order to save Atlantis as well as the planet, Sheppard comes up with an idea. The ZPM is transferred to the Daedalus and the ship moves into position between the sun and the planet. When the coronal mass ejection begins, the shields aboard the Daedalus (boosted by the ZPM) successfully deflect the emission away from the planet, the Daedalus sustaining only minor thermal damage in the attempt.