Name this weed please

Ages since I've posted here and I feel bad, but May is always a busy month for me and I can't seem to get a word typed in edgewise. I'm really grateful for you readers who stop by here when I post, and I'll try to keep up better now!  I have a lot of spring flower-riot type photos, since it is that quite magical time of year, but for the moment, here's a quick, if embarrassing, ID I need help with!  You weed-identifiers, please help me out. This crazy thing started growing about a year ago and it had a monocotish appearance at first--just a few blades--so I thought it might be one of the bulbs I'd planted months earlier, just behind schedule for whatever reason. It stayed in a holding pattern for months and months and finally started growing tall this spring. It finally developed more slender leaves and stalks almost 5 feet high, and when it developed buds they looked awfully weed-like, so I was disappointed. I'd planted an Ascelepias speciosa roughly in that spot early in the garden's life, but it seemed to die (for some reason I have had no luck whatsoever with Asclepias) and even though the leaves weren't like that plant's, I sort of wondered if it were maybe some other Asclepias, mislabeled from the nursery. At some point when I wasn't looking one of the buds not only flowered but moved right on to become a huge crazy seedhead. That had to come down before spreading over the entire neighborhood.  Anyone know what it is? Most weeds come in dozens or hundreds or thousands, but this is the only one of its kind out there that I know of.

Thanks for any info!  Pictures of happier, non-impostor plants to follow soon!