I can actually plug this line in... and, using the other end, plug another strand on and it works.
I've literally pulled EVERY bulb and have re-installed every bulb (in different sockets - pulled all of them, filled all sockets again with that bulb mix). Same outcome.
We can't locate anything that came with the tree -- although, we're nearly positive that it's here somewhere. That said, rather than trying to find a replacement branch set, it's going in the freaking dumpster. I'm sick of this shiz.
Thanks for the attempted help, folks.