Starlight knows she's not supposed to travel back in time. But when Starlight's been drinking she tends to forget things.
Now she and Spike are trapped in the past with no idea of how they got there or how to get back. Holding out hope that Twilight or someone else will rescue them they work hard to not cause any changes to the timeline. Or at least that's what they should be doing. Starlight Glimmer might have some other ideas.