Verbascum thrapsus is the proper name, it's a common enough cottage garden plant, and often pops up unexpectedly as the seeds are tiny, and very light, and can spread considerable distances, and can survive in the soil for many years.
The first sign is a rosette of fat, furry, grey-green leaves. This rosette gets bigger... and bigger... and bigger... and eventually a flowering spike shoots up from the middle and ascends to five feet high, or more.
I must admit that I'm a bit soft-hearted when it comes to weeding them out, and if they are in a place where they won't crowd out other, more valuable plants, I will let them grow on.
Earlier this year I left one, it grew a huge towering flower spike, then we some very hot days and, lacking slightly in water, it began to droop. Then it rained.
Look what happened!
As an aside, in the background you can see a rose-covered fence with clematis growing through it: something of which I disapprove strongly, as it makes it impossible to prune either of them properly.
However, for just a week or so, it makes a glorious show, doesn't it?
Returning to the Mullein, I found another one doing strange things in another garden:
Both of these are fairly normal variations on a theme, nothing sinister about them, but I think they are quite interesting.
As the rosette of leaves at ground level is so dense, I will usually remove the leaves as they start to get a bit tatty, or if it is threatening to smother something more desirable, I simply cut all the leaves right back.
It doesn't seem to slow the plant down at all!
If you don't want it to seed everywhere, make sure to chop off the flowering spike as soon as it finishes flowering. And if you do want it, remember that it's a biennial, which means that the first year will just be a small rosette of leaves, and it won't flower until the following year.