The first details on “Critical Mass” have arrived! Written by staff writer Carl Binder, the mid-season episode of Stargate Atlantis will go before cameras later this summer, and will air in the second half of Season Two in early 2006.

Beware of spoilers for this episode and for the season premiere, “The Siege, Part 3,” in the report below!

The city of Atlantis has laid hidden from the Wraith for months, but all of that may be about to change.

Rodney McKay introduces Colonel Sheppard to Dr. Alison Seagle, a scientist who has just arrived on Atlantis from Earth (via the Daedalus, now making regular trips between Earth and the Pegasus Galaxy). Seagle is joining McKay’s team, and Sheppard irritates McKay by flirting with her.

The Atlantis base is now making weekly contact with Stargate Command on Earth, dialing in to relay updated information.

Meanwhile, Teyla asks Dr. Beckett to join her in a trip to the mainland. She is greatly concerned about her old Athosian friend Charin, who apparently needs medical attention.

The team is now able to hide the city of Atlantis with a massive cloaking device, but someone has set off a beacon. The Wraith have been alerted, and two Wraith Cruisers are detected moving toward the planet by long-range sensors. Though Sheppard once had an affinity for Dr. Seagle, he now has reason to treat her with an air of suspicion. Her strange behavior even causes McKay to suspect that she has betrayed them.

Guest characters in “Critical Mass” include Dr. Alison Seagle, Colonel Steven Caldwell (Mitch Pileggi), and Dr. Radek Zelenka (David Nykl).

Stay tuned to GateWorld for the very latest!