One of the longest story-lines in the Star Trek universe is definitely
the one about those almighty beings called the Q. They, together with their
most well known representative Q (portraid by John de Lancie), had their first
appearance right in the first episode of the second Star Trek show: Star Trek:
The Next Generation and since then appeared in not only a couple of other
TNG-episodes (including the last one) but also in DS9 and Voyager. With Q & A Keith R.A. DeCandido extends the world surrounding the Q Continuum
with another story that brings Q again down onto Captain Picard and the crew
of the Enterprise to finally explain some things.