Singularity

The SG-1 team travels to planet P8X987, they discover that a strange disease has killed everyone on the planet. except a girl named Cassandra. As O'Neill and Teal'c remain on the planet to observe a black hole.

Carter and Daniel return with the girl, and are shocked to discovers that chest pains Cassandra is experiencing are caused by a somthing metallic growing around her heart. It seems that the Goa'uld planted the device and used the girl to carry the "bomb", to destroy the Stargate on Earth.