This plant showed up in our garden this spring. It just began blooming a couple days ago.
Here are buds, getting ready.
Is this some kind of phlox? The plant itself is about 2 1/2 feet tall and looks quite civilized.
You guessed right! This is Phlox paniculata, or tall phlox.
These grow wild in my woods, but there are many cultivars developed for garden use. That is a very nice volunteer plant, and I know a good many people who would be envious of such a healthy volunteer specimen in their garden.