The episode begins with SG-1 preparing to go to a planet to observe a black hole, which is made possible by an impending eclipse. As they are heading out, Teal'c is asking Carter about black holes, when O'Neill suddenly explains the phenomena surprisingly well. Daniel and Teal'c are shocked, as Carter comments, "You didn't think the Colonel had a telescope on his roof just to look at the neighbors did you?" As they arrive through the gate, they find that the welcoming committee is unexpectedly absent. They proceed towards the outpost, when they come across a dead body lying by the road. Quickly putting on gas masks and gloves, they explore the rest of the town, to find that everyone is dead.
They dial Earth to report the situation, and a hazmat team is sent through to check them out. Fraiser finds that they escaped contamination, and it must have been some sort of disease that wiped out the people. The team returns in full hazmat gear to document the dead and decontaminate the area. As they are tagging bodies, they come across a small child hiding in the bushes. Surprisingly, a young girl survived. They comfort her and bring her back to the outpost.
Fraiser finds that the girl has the presence of naquadah in her blood, which may be why she survived. As they prepare to leave, Carter mentions that they should continue with the observation of the black hole, as this is perhaps their best opportunity to do so. Jack notices that the girl has formed a bond with Carter and sends her back, while he and Teal'c will handle the observation. Carter agrees to stay with the girl and keep her company, as she seems to not want to be alone.
Fraiser and Daniel meet with Hammond, and explain that the disease may have been caused by bacteria brought from Earth that mutated and wiped out the people. But they might have the cure in the girl, who may hold the key to treating other diseases. Carter and the girl have kept busy drawing pictures in the room. At one point, she draws a picture of her standing in a field crying, with dead bodies everywhere. Sam then draws herself standing next to her, and says "You're not alone anymore." When Carter tries to leave at some point, the girl stops her, and she finds out the girl is named Cassandra. Cassandra then complains that she's feeling pain in her chest, so they go get checked out by Fraiser. They believe it's low potassium, when she suddenly collapses. Fraiser hears a strange noise in the girl's heartbeat.
They perform an X-Ray, and find that some sort of object has appeared next to the girl's heart. They perform a biopsy, and as the doctor reaches and begins to take a sample of the object, it stops her heart. Luckily, her heart restarts once he pulls the scope back out. Using the samples obtained, they find that one half of it is made of naquadah and the other iron and potassium, separated by a thin layer of fatty tissue. Carter then performs an experiment to find out what would happen if the two sides touch. What results is a huge explosion, and Carter only used microscopic samples. Hammond decides that they need to return the girl through the stargate, before the device in her chest explodes.
Meanwhile, Jack and Teal'c wait for the eclipse to happen, and they notice a strange blip appear in space. As they zoom in, they find that it is a Goa'uld ship. Teal'c says that they must leave NOW. As they run for the gate, Teal'c talks about one Goa'uld, Nirrti, who used a trick to destroy his enemies. He would send an ambassador through the gate to meet with his enemy. As the ambassador left to return through the stargate, the gate would be destroyed somehow. Jack realizes that the girl is probably the same tactic. He and Teal'c rush to the gate with death gliders firing on them.
Back on Earth, they are preparing to dial the gate and go back. As they wait for the sequence to complete, Cassandra collapses again. However, before they can dial out, Jack and Teal'c dial in from the planet and leap through amid a cloud of debris from a glider shot. Jack tells them to get the girl away from the gate! Recounting Teal'c story, they determine that the girl was this same trick, since the Goa'uld knew they would discover her among the death and bring her to Earth. They decide to take her to an abandoned nuclear facility nearby as they figure they have 30 minutes until her time is out.
When they arrive, Carter goes down the elevator with Cassandra, and is told she only has a few minutes to turn around and come back up. On the way down, Cassandra wakes up, and Carter is very emotional trying to keep her calm. They reach bottom and Carter takes her over into a small room. She tells her that she has to go for a few minutes, but she'll come back. As she starts up the elevator, she can't handle it anymore, and returns to the room. O'Neill uses the intercom to ask her why she went back, and Carter said she was awake and she's staying with her. Tense minutes pass as the time runs up...with no explosion. Turns out they're both fine, it didn't happen.
Apparently once she got far enough away from the gate, the bomb started to be reabsorbed by the body. They decide that Fraiser will adopt her for the time being, and they come up with a cover story about how she came from Toronto. The episode ends in a park with Jack giving her a dog and Carter teaching her about swings as the rest of the team smiles.
Tropes used in "Singularity" include:
- Bread, Eggs, Breaded Eggs:Daniel: What exactly are we going to see during this eclipse? I mean, it's black, it's a hole...O'Neill: Maybe it's a black hole?
- Cute Mute: Cassandra, initially. She does speak later on, though.
- Dangerous Phlebotinum Interaction: "Singularity" shows that in addition to the main way naquadah is explosive, it reacts explosively with an alloy of iron and potassium (we're talking two microscopic amounts of each obliterating a whole room, including the camera on the other side of it), a fact used by Nirrti to turn a girl into a walking weapon of mass destruction.
- Double Take: Daniel does a pretty epic one when O'Neill (correctly) identifies the accretion disk of a black hole.Carter: You didn't think he had a telescope on his roof just to look at the neighbours, did you?
- Foreshadowing: While we won't actually meet her onscreen until "Fair Game", the introduction of Nirrti begins expanding the Goa'uld's ranks beyond the Egyptian pantheon. It also establishes other System Lords beyond Apophis are starting to take an interest in Earth (and in Nirrti's case, viewing them as a legitimate threat). This will start paying off dividends over the next two Seasons.
- Heroic BSoD: Carter completely breaks down after leaving Cassandra in the basement of the nuclear facility, before she has a change of heart and goes back to wait with her.
- Hidden Depths: O'Neill demonstrates both a knowledge of and an interest in astronomy - which as Carter points out shouldn't be all that surprising considering he has a telescope on his roof.
- Improbable Infant Survival: Played straight; the bomb doesn't detonate and is reabsorbed into Cassandra's body, allowing her to be Happily Adopted by Janet Fraiser and live a normal life on Earth.
- Meaningful Echo:"You're very brave, remember?"
- Nice Job Breaking It, Hero: At first subverted and then played straight; Fraiser initially hypothesizes that the SG team may have unwittingly brought the disease to the planet of the week, but in actual fact it was deliberately introduced by the Goa'uld. And then the heroes realize that they activated the bomb inside Cassandra by bringing her through the gate in the first place, and then helped it along by giving her iron supplements and introducing an electrical charge from the defibrillators.
- Nightmare Fuel Colouring Book: Cassandra's painting of a girl surrounded by stick figure corpses.
- Oh, Crap!:
- Carter and Fraiser, on hearing the bomb ticking inside of Cassandra.
- O'Neill and Teal'c, when they realize that Nirrti's ship is orbiting the planet.
- Parental Substitute: Both Carter and Fraiser for Cassandra. It's made more obvious with Carter, but Fraiser is the one who eventually ends up adopting her.
- Sole Survivor: Cassandra is the only survivor on a planet where the rest of the local population and a visiting SG team has been wiped out by The Plague.
- Trojan Horse: Nirrti deliberately uses Cassandra as one in an attempt to smuggle a bomb through to earth without anyone noticing before it's too late to do anything about it; Daniel invokes the trope by name.
- Why Am I Ticking?: Cassandra, due to having a naquadah bomb inside her chest.
