Season 1 Episode 15 - Singularity

The SG-1 team travels through the Stargate to planet PX3-987, where another SG team has been making preparations to observe a black hole. But, when they get there, they discover that a strange disease has wiped out everyone on the planet. Everyone that is, except a little girl named Cassandra. As O'Neill and Teal'c remain on the planet to observe the black hole, Carter and Daniel return to Earth with the girl. Carter grows especially close to Cassandra and is shocked when she discovers that the chest pains Cassandra is experiencing are caused by a metallic device growing around the child's heart. Daniel and Carter are certain that the Goa'uld planted the device as part of a scheme to destroy the Earth's Stargate. As the time bomb ticks down, Carter is torn between her love for the little girl and her knowledge that the child is being used as a Trojan horse by the Gou'ald.