Sorry if this has been asked before, but I had a thought after watching Yesterday's Enterprise...
The episode starts in a timeline where Tasha Yar existed once, then died. When Enterprise C comes through the rift, we enter a timeline where Tasha existed once but never died. When Enterprise C returns to the past, the timeline returns to "normal", as confirmed by Guinan, and the series continues on...
But, at the end of the episode have we not entered another alternate timeline? It's not the same timeline as at the beginning of the episode. Tasha now existed twice, died twice and had a daughter. The episode emphasises throughout that Enterprise C must return and everything happen EXACTLY as it did before in order to fully restore the timeline. However, as Yar goes back and survives, this doesn't happen. The timeline does return to a non war state, but it's different due to the alternate Yar's survival and the later appearance of her daughter.
So, does that mean that the last we see of the "original" timeline is at the start of this episode, and from the end of the episode onwards we enter an alternate timeline based on the choice made by Picard and Yar to allow her to go back with Enterprise C? Technically the timeline we follow for the rest of the show shouldn't have happened either. It was only because Picard allowed Yar to go back (assuming she would just die with the others) that another alternate timeline was created which erased the original timeline at the beginning of the episode (and the timeline we'd been following up until that point for the first two and a half seasons).
The episode starts in a timeline where Tasha Yar existed once, then died. When Enterprise C comes through the rift, we enter a timeline where Tasha existed once but never died. When Enterprise C returns to the past, the timeline returns to "normal", as confirmed by Guinan, and the series continues on...
But, at the end of the episode have we not entered another alternate timeline? It's not the same timeline as at the beginning of the episode. Tasha now existed twice, died twice and had a daughter. The episode emphasises throughout that Enterprise C must return and everything happen EXACTLY as it did before in order to fully restore the timeline. However, as Yar goes back and survives, this doesn't happen. The timeline does return to a non war state, but it's different due to the alternate Yar's survival and the later appearance of her daughter.
So, does that mean that the last we see of the "original" timeline is at the start of this episode, and from the end of the episode onwards we enter an alternate timeline based on the choice made by Picard and Yar to allow her to go back with Enterprise C? Technically the timeline we follow for the rest of the show shouldn't have happened either. It was only because Picard allowed Yar to go back (assuming she would just die with the others) that another alternate timeline was created which erased the original timeline at the beginning of the episode (and the timeline we'd been following up until that point for the first two and a half seasons).