SG-1 goes to a planet where a prior SG mission has set up a telescope to observe the eclipse of a black hole. However, when they arrive, they find that everyone on the planet--both SG-7 and the indigenous population alike--are all dead, victims of an epidemic. One young girl, a native of the planet, is the lone survivor who is mysteriously unaffected by the plague. The girl, whose name turns out to be Cassandra, develops a strong, close bond with Sam. Sam and Daniel bring Cassandra back to Earth, while O'Neill and Teal'c agree to remain on the devastated planet and man the telescope so they can still record the eclipse of the black hole. Once back at the SGC, Cassandra complains of not feeling well, when Sam takes her to see Dr. Frasier, they discover that she has a Goa'uld device within her chest.

The device appears to be mostly organic in nature, which started assembling itself shortly after Cassandra arrived on Earth. Any attempt to remove or tamper with it will kill Cassandra. The device is deteriorating on the cellular level, and once the two components within the device meet, it will set off a catastrophic explosion. Sam, Daniel and Dr. Frasier realize that the entire plague was a set-up by the Goa'uld. They slaughtered the population of an entire planet, along with an SG team, and planted the young girl with the bomb, knowing full well that SG-1 would take pity on her and bring her to Earth, where the bomb within her would form and explode. Yet they soon learn that they can't bring her back through the stargate, for the device within Cassandra is designed to explode the moment she goes near the gate. However, at the rate the device is breaking down, it appears that it will explode anyway. And so an extremely grim decision is made, one which tests Carter's resolve in a nerve-wracking climatic scene.

Singularity is a marvelous episode that shows just how ruthless and insidious the Goa'uld can be. It starts out as a seemingly regular SG-1 episode at first until we are stunned by the revelation of the Goa'uld's vicious treachery. And just when we think we've seen it all, we're walloped by this final, heart-rending twist. This episode gives Amanda Tapping a great showcase for her acting talent, and she's terrific. Her final scenes with Cassandra are both agonizing and poignant to watch. This was the episode that ultimately won me over as a fan of Samantha Carter--thanks to the brave, selfless manner in which Carter ultimately deals with this unbearable situation.