Here we get another line that doesn't seem to tell us much. "And which" is just a clue that tells us we're going to hear something more about these plums.
We're starting to suspect here that the speaker is choosing his words very carefully. It's like he's walking on his tiptoes, careful not to use a word that's going to step in the wrong place.
While, of course, most poets are very careful about their words, we think there's another reason why something like "and which"—or line three from this poem, "that were in"—would be on lines of their own.
Even though this poem tells us it's "just to say," the speaker has some sort of stake in what he's saying, or he wouldn't be taking so much time to say it.
you were probably
Oh, so these weren't just any plums. They were someone else's plums.
That means there's another person involved in this poem. Maybe this "you" has something to do with why the speaker is being so careful and taking so much time with his words.
From here on out, we're going to refer to "you" as a female. We don't know what gender "you," is, but we'll pick one just to make things less confusing, and because if we read the poem biographically, we'd guess the "you" was Williams' wife.
With the word "you," we can guess that this poem is written to someone specific. The poem, introduced with the idea that "this is just to say," is meant to be read by her. Sure, this means us, the readers, too, but we're guessing it's mainly addressed to someone the speaker knows in real life.
We also find out that she was "probably" doing something with these plums. So the speaker knows her well enough to guess her intentions.
With only one six-letter word, this is the shortest line of the whole poem.
But it actually gives us a lot of information, instead of longer lines like "that were in" and "and which," which just help to situate us.
We find out with this line that the speaker has eaten the plums, which he thinks someone else was saving. They were, after all, in the icebox, and not just left out.
So, maybe, we start to think, these lines are so spaced out and slow because the speaker feels a little guilty. He's trying to come up with the best words to apologize for eating these plums, even though he's not absolutely sure that they were being saved by someone else. It's just a hunch.
Here we find out that the plums were probably being saved for breakfast. So this gives us a bit of a scene.
The speaker is either going to bed late, after everyone else, or he's waking up early, before everyone else. He looks around, tries to find something to eat, and can't resist the juicy plums in the icebox.
This line also hints that the speaker lives with "you." Maybe "you" is his wife, his mother, a sibling, or simply his roommate. Either way, they're around for breakfast, and were there to stow the plums away.
We feel a little bit of the speaker's guilt here—the person who was saving the plums might wake up and have nothing to eat now. The speaker, we're guessing, will be gone when she awakes, so he's writing this to explain what happened to the plums.
We know how Shmoop would feel if a fruit bandit stole our plums in the wee small hours of the morning, but how would you feel?