Exactly what it says. What I remember of the story is that Luna finds out in the present that the destruction of their old castle killed some ponies. One particular pony as well, and she tries to send him away, only for him to return and save her from getting crushed, showing the future can't be changed that much. Celestia notes when she gets back that Luna's attempts to convince her past self, though failed, gave her hope to reunite. Luna tries several ways to stop herself, but it's the failure to save the guard that I remember the most. That, and that it was revealed that guard/stallion is the ancestor of Prince Blueblood.

Please someone have read that, this is driving me CRAZY!!!