Critical Mass

"Critical Mass" is the thirteenth episode of the second season of Stargate Atlantis.

Plot
John Sheppard meets with Radek Zelenka in the Gate room aboard Atlantis. Zelenka is being sent to M7G-677 (the planet with all the kids) to repair their EM-field generator. Zelenka however, is not happy; he does not like children. Meanwhile, Dr. McKay radios Sheppard, Weir and Ronon to show them two Wraith cruisers he has discovered near Atlantis, but not on their way to the city. They are actually shooting at each other.

At Stargate Command, General Landry is visited by Agent Malcolm Barrett, who informs him that the Trust is now controlled by the Goa'uld and that they have planted a bomb in Atlantis, which will explode the next time the Atlantis Expedition dials Earth – which is scheduled for later that day. Landry orders Dr. Bill Lee to quickly find a way to warn them.

Back in Atlantis, Rodney meets Lt. Cadman again but feels uneasy around her, due to a previous situation. Meanwhile Teyla asks Dr. Beckett to accompany her to the mainland to examine her old friend Charin who is very ill. They do so and Charin asks Teyla to prepare the Ring Ceremony, which is used to celebrate the natural death of a person.

On Earth, Dr. Lee discovers a way to contact Atlantis by using a small planet at the edge of the Milky Way galaxy and the Daedalus as relay points. They are able to contact Daedalus, which then relays the message just in time to stop the dialling process is completed. Afterwards, McKay disconnects the ZPM while teams search the city for explosives. Unfortunately, they find nothing. The Daedalus is also called back to the city to help find the saboteur, but before this they relay another message to Atlantis. Suddenly, Atlantis' DHD begins to dial Earth on its own but can not establish a connection due to insufficient power. As if in response, the city's transmitter sends a beacon to the nearby Wraith cruisers, alerting them to Atlantis. The city will have to be cloaked again.

Later, Weir begins an inquiry to find out who planted the bomb and when the Daedalus returns, she questions Peter Kavanagh who was aboard the ship. Kavanagh however, reasons that he is the main suspect because Weir is being driven by emotions. Next, Weir questions Dr. Novak but during the inquiry, Rodney has made a discovery: there is no bomb. In truth, the fail-safe of the ZPM was deactivated and blocked with a code, thus causing a massive overload in the device if enough energy is supplied. They now have a dilemma, they need the ZPM to cloak the city. Weir questions Kavanagh a second time, but he does not say anything. She then discusses the situation with the senior staff and Ronon hints at torturing the code out of Kavanagh as a quick solution; however, Kavanagh immediately faints under threat from Ronon.

Meanwhile, Teyla evacuates her people back to Atlantis and also tries to persuade Charin to let herself be treated by Beckett, but Cherin wants to accept her death. She is later brought to the Atlantis infirmary and although Dr. Beckett wants to treat her, Teyla persuades him to let her die.

The Wraith have appeared above Atlantis and the city must be cloaked to avoid detection. Suddenly however, the inertial dampeners of the city activate, which causes the ZPM to overload. Atlantis will be destroyed in roughly 30 minutes. The senior staff then decides to evacuate the people of Atlantis to a planet in the same solar system where they found the crash-landed Wraith ship. While all the preparations are done Teyla and Beckett conduct the Ring Ceremony for the dead Charin, at which Teyla sings for her.

10 minutes before the ZPM overloads Lt. Cadman informs Weir and McKay that she discovered that Steven Caldwell was responsible for everything. When they question him, they find that he is a Goa'uld. Ronon subdues Caldwell and Sheppard uses a Taser on him, allowing Caldwell to momentarily take back control and give them the fail-safe code. Rodney enters it into a console and the overload is averted.

Later, Rodney informs Beckett and Cadman that Hermiod plans to use the Asgard beam to remove the Goa'uld symbiote from Caldwell. Zelenka has also returned, face painted, hair beaded, and in a foul mood. Weir talks with Sheppard and questions her decision in handling Kavanagh, but Sheppard reassures her that she only did what she had to do.