Stargate Atlantis: Season 2, Episode 13

2 - 13: Critical Mass

On Earth, a galaxy away from Atlantis, Gen. Landry receives intelligence that The Trust, a group of conspirators controlled by the evil Goa'uld, have learned of the Ancient city and sent an undercover operative there to plant a bomb. Details are sketchy, but any use of Atlantis's stargate to dial Earth will somehow trigger the explosion.