SG-1 is destined for the planet Hanka to observe a singularity which will be evident during an upcoming total eclipse. After initial contact with the native population by SG-1 three months earlier, SG-7 established an observatory on the planet. When SG-1 arrives on the planet's surface no member of SG-7 is present to meet them. This appears odd, but not unreasonable.

Proceeding in the direction of the observatory, they discover a man lying by the side of the road. Jack rolls him over and they immediately realize that he has died of an unknown disease. They don protective gear and continue into the village and to the observatory where they find all members of SG-7 and inhabitants of the nearby village are dead of the same disease.

After returning to Earth and undergoing decontamination, they return to Hanka, this time in full bio-hazard gear, to investigate and tag the dead. Daniel reminds them that when they made initial contact the inhabitants had warned them something dire would happen at the time of the eclipse. SG-1 had assured the people that *nothing* would happen. It appears the people were right. As they continue to tag the bodies, Sam bends to place a tag on a woman. When she turns to go she hears rustling in the tall grass. They are able to lure out a young girl who appears to be perfectly healthy.

A thorough physical by Dr. Fraiser reveals that the girls is not infected but does have traces of naquada in her blood. Fraiser also reports that the entire area around the Stargate is infected with the disease causing bacterial agent. All SG teams are to be pulled back to Earth but Sam protests. Someone, she argues, should stay and observe the eclipse. They owed that much to the deceased members of SG-7. Jack and Teal'c volunteer to remain. Sam, Daniel and the little girl return to Earth.

The final death toll on Hanka is 1432. Dr. Fraiser thinks the bacteria that caused the epidemic might have been introduced to the planet by SG-1 during their first visit. But, Hammond wants to know, why SG-7 didn't warn SGC something was wrong when the first signs of the epidemic became evident.

Sam has gained the little girl's confidence enough to discover that her name is Cassandra. The captain says she needs to go away for a short while, but before she can, Cassandra has a sudden pain in her chest. Back in the infirmary Dr. Fraiser says she can find nothing wrong except a sharp drop in the child's blood potassium levels. This, she informs Sam, can cause heart irregularities.

As Sam and Cassandra start to leave, Cassandra collapses, and within seconds is in cardiac arrest, probably due to the low potassium levels. Dr. Fraiser manages to save Cassandra but not before she is forced to shock her heart to restart it. As she listens to Cassandra's chest she hears a very strange sound in the girls chest cavity and immediately orders an x-ray.

Exploratory surgery allows Dr. Frazier's team to get a closer look at the object as well as a sample from its surface, but in the process Cassandra's heart stops again, only to re-start on its own after the exploratory scope is removed.

Analysis of the material shows that there are two objects in Cassandra's chest with a layer of fat between them which is slowly decaying. One object is composed of naquada and the other of iron and potassium. Potassium is very volatile. Combined with naquada the result is highly explosive. There is enough material in Cassandra's chest to cause a nuclear explosion sufficient to destroy the Stargate and the mountain that houses it should the two objects come in contact. And the fat layer that separates the two objects is slowly, but regularly decaying.

Meanwhile in the observatory on Hanka, Jack and Teal'c discover an object hiding near the singularity. It is a Goa'uld attack vessel.

Daniel concludes that the Goa'uld have set them up. They knew the humans would bring Cassandra to earth. They designed the device in Cassandra's chest so our own medical technology would activate it.

Cassandra is a Trojan horse and they have one hour and 52 minutes before the device detonates. Sam is outraged and hurt, she has also grown much fonder of Cassandra than she knows she should. Daniel tells her that to the Goa'uld Cassandra isn't a little girl, she's just cheap tool to get to SGC. General Hammond has no choice buy to send Cassandra back, through the Stargate to Hanka. SG4 is to escort her.

Meanwhile, on the planet's surface Teal'c and Jack are under attack by a Goa'uld fighter, as they run for the Stargate. Teal's tells Jack that the vessel they saw near the singularity belongs to a Goa'uld named Nehrti, an enemy of Apophis. Nerhti had sent an emissary of peace to negotiate a treaty over a Stargate Apophis had taken control of. It was a ploy . The Stargate was destroyed when the emissary passed through the gate. O'Neill realizes that the girl is also a ploy.

SG-4 is ready to take Cassandra through the Stargate back to Hanka when she slips into a coma. Before they can get the gate operational Jack and Teal's fly through warning them to get Cassandra away from the gate. The Goa'uld kept SG7 from coming back to warn them of the plague. It was all part of their plan. The earth Stargate will be destroyed if Cassandra goes through it.

They have no options but to send Cassandra to an abandoned nuclear facility hoping to contain the blast when the device inside her detonates.

Sam escorts her to the bottom levels, thirty floors of solid rock below the surface. But Cassandra wakes up before they reach the bottom. Carter is crying at this point. She takes Cassandra to a secure area, but instead of abandoning her, she chooses to stay, seals the door, then waits.

As the clock ticks down there is no explosion. How did Sam know? She realized that Cassandra had collapsed when she was in proximity to the Stargate and revived when she was removed some distance from it. The Goa'uld plot won't work if Cassandra doesn't go through the gate.

Later on, the SG-1 team are outside with the girl. Daniel asks if Sam knew that the device wouldn't blow and Sam replies she didn't know, but felt it. Teal'c said that perhaps it was a mother's instinct. Sam smiles at Teal'c's hint but reveals that Cassandra's going to be adopted by Dr. Fraiser and that device in Cassandra's chest is shown to be shrinking.