SG-1 travels to the planet Vyus, where an event known as the Vorlix has left the entire population suffering from a case of mass amnesia. Daniel experiences a mutual attraction with one of the planet's inhabitants, a gifted scientist named Ke'ra, but the rest of the team comes to suspect that she may not be what she appears when it turns out that the memory loss is the result of an experiment by Linea.
"Past and Present" provides examples of the following tropes:
- Amnesiac Dissonance: Ke'ra recovering her memory of being Linea immediately leads to conflict between such contrasting personas and prompts her to attempt to kill herself so that she doesn't hurt anyone.
- Amnesiacs are Innocent: Linea was a genocidal Mad Scientist prior to losing her memories, but after the Vorlix she becomes Ke'ra, a genuinely good person who dedicates her life to helping others.
- Can't Have Sex, Ever: Ke'ra indicates that the people of Vyus have been voluntarily celibate since the Vorlix, to avoid accidentally sleeping with the wrong person (such as a sibling or enemy instead of their spouse). This is part of why Ke'ra is so aggressive in trying to jump Daniel; she hasn't had sex in over a year and she knows that, since he's an off-worlder, there's no way for Daniel to be someone she shouldn't be sleeping with.
- Childless Dystopia: There are no children on Vyus due to the use of a pesticide that adversely affected fertility. Before the Vorlix, the entire population was old. The Vyans note that they have pictures of their seemingly missing elders (who are actually themselves) but none of the just-as-absent children, even though they remember what a child is and that they must have been children at some point.
- Dogged Nice Guy: Orner in a faintly creepy sort of way towards Ke'ra, whom he insists on believing he was married to before the Vorlix, before he takes the cure and remembers that he is actually married to Layale.
- The Dreaded: Linea, with SG-1 speculating that what happened on Vyus was her attempt at a new form of genocide (as the Vyans are facing a crisis due to a lack of agricultural knowledge etc). As it turned out, it was a relatively benign experiment on her part - she was just trying to make herself younger again, instead turning herself and everyone else into a rejuvenated amnesiac. Of course, there were the two dead test subjects in her lab, and the reason she was trying to make herself younger was so that she could go on destroying worlds...
- Fatal Attractor: An episode after he almost gets his brain fried by the alien parasite possessing his wife, Daniel gets involved with a mass murderer who was once known as the 'Destroyer of Worlds'. Granted, neither of them knew who she really was at the time.
- Fighting from the Inside: Ke'ra struggles to retain control after taking the cure, though she believes the Linea personality will eventually take over and revert back to her old ways.Ke'ra: There is a part of me that cares for you, Daniel, more than I have ever cared about anyone... But there is this other part of me that would gladly watch you die!
- Fountain of Youth: It turns out that not only have the Vyans lost their memories, they've also regressed in age by several decades. The whole situation is actually the result of Linea experimenting with ways to make herself young again, but thanks to a freak lab accident it ended up de-aging the entire population of the planet instead of just her, and the amnesia was an unintended side-effect.
- I Was Quite a Looker: Judging by Ke'ra, Linea was quite the blonde bombshell in her youth.
- Identity Amnesia: None of the Vyans have any memory of who they were prior to the Vorlix (and have all given themselves new names since they can't remember their original ones), and as evidenced by Ke'ra, at least some of them are now completely different people, while retaining core knowledges like the language and in Ke'ra's case biochemistry.
- Internal Reveal: From the perspective of the people of Vyus, the mere fact that Teal'c remembers his father affirms that he's not a native, as none of them remember their own parents after the Vorlix.
- Interrupted Suicide: After taking the cure and remembering that she really is Linea, Ke'ra intends to kill herself by releasing a deadly poisonous gas in a closed room instead of risking Linea taking over. Daniel stops her by refusing to leave the room and persuading her to erase her memories all over again instead.
- Like an Old Married Couple: Orner and Layale, who spend the entire episode bickering and sniping at each other. Made heartwarming at the end when they remember that they actually were married for 43 years.
- Mad Scientist: Both Ke'ra and Linea - the former benevolent, the latter... not so much. Even with her Linea memories locked away, and only a latent knowledge of genetics and bio-chemistry, Fraiser describes Ke'ra as being "so far ahead of us it's frightening."
- Meaningful Echo: Ke'ra repeats the phrase "all debts have now been paid", which is part of the message that Linea sent through the base computers after making her escape in "Prisoners", and also "you've been very kind", which echoes the "thank you for your kindness" in the same message. This causes Daniel to realize that she has taken the cure and now remembers who she is.
- The Medic: Ke'ra uses her latent knowledge of biochemistry to fill this role on Vyus.
- Nice Job Fixing It, Villain: Linea's experiment getting out of control not only neutralizes herself as the Destroyer of Worlds but also solves Vyus's fertility problem. This also qualifies as Gone Horribly Right from Linea's point of view.
- Not That Kind of Doctor: Daniel again, when he first meets Ke'ra and gets somewhat tongue-tied:Daniel: So, I'm guessing you were — are — a doctor?Ke'ra: It seems to me that before the Vorlix I helped others. That is all that I sense. Are you?Daniel: Uh, a doctor? No, no. Um, well — yes, of... I'm a doctor of archaeology, um, I study the past.
- Note to Self: Ke'ra claims that she was able to deduce the Vorlix happened quickly because if the Vyans had advance warning about their fate, they probably would have left themselves some kind of message explaining what happened.
- Oh, Crap!: The team's reaction when they realize Linea is involved. Fraiser also gets this later on when she notices that Ke'ra has stolen a sample of the cure.
- On the Rebound: It's implied throughout the episode that Daniel's fling with Ke'ra is a somewhat misguided attempt to move on from Sha're's death.
- Sequel Episode: To "Prisoners".
- "Shut Up" Kiss: Ke'ra kisses Daniel to halt his nervous rambling after she first makes her intentions towards him clear.
- The Sociopath: Linea, as per usual. Although given that Ke'ra is sane, she apparently was not a clinical sociopath, or her messed up brain chemistry would presumably still be a problem even with amnesia. Which makes it all the more mysterious how Linea originally got that way.
- Steampunk: The civilization on Vyus seems to have a Victorian aesthetic going and a similar level of technology too.
- Suddenly Shouting: When Ke'ra's struggling to keep her Linea personality under control.
- Who Names Their Kid "Dude"?: When the introductions are made at the beginning of the episode, Orner questions what kind of a name is "Teal'c". Teal'c takes it in stride:Orner: Who'd give you a name like "Teal'c"?Teal'c: It was given by my father. It means "strength".
- Zeppelins from Another World: One is briefly seen in the sky on Vyus.
