People seem to get these two confused, so let’s clear the waters, shall we?
Premise: the basic idea of the story
Plot: what happens in a story
They are not the same, though they are related. I’ve seen it said that the premise is your initial idea, what drew you to a story, what made you want to write it. My premises tend to be things like “girl saves friend from evil dimension by use of their shared locker” or “pirate will sacrifice everything to raise her lover from the dead.” The premise gives you an idea, but it, by itself, is not enough to support a story.
The plot is the events that happen in a story. So the plot of the above pirate story, summarized very succinctly, would be something like, “Pirate gathers crew for an ocean voyage to find a lost artifact capable of bringing the dead back to life, knowing they may perish in the attempt. Along the way, the crew bonds over shared stories and dangerous encounters. When they reach their destination, pirate realizes she has grown and no longer needs said artifact.”
See the difference?
Now, of course, the plot is every event in a story that contributes to the main storyline (events that contribute to less important storylines are part of the subplot).
So. Premise = idea, Plot = series of events.