So, in the opening scenes of A New Hope Princess Leia leaves a message for Obi-Wan Kenobi, asking her trusty plot-exposition droids to deliver it to an old recluse on a distant planet.
It is, at this stage, a closely guarded secret that
Darth Vader is Luke Skywalker's father
and even more closely guarded that
Darth Vader is Princess Leia's father
The recluse no longer goes by the name Obi-Wan, and claims he hasn't since before the birth of Leia and Luke.
- Leia is chasing shadows, asking droidle-dee and droidle-dum to find a man she's heard of in legend, in a place he's rumoured to be.
- Leia has had some kind of contact with Old Ben, he has used the name Obi-Wan in her lifetime, and he's a big fat liar (see this question).
- She's got credible information from someone who was around beforehand that there's a bloke called Obi-Wan on Tatooine (a whole damn planet for the droids to search!?!), and that he can help.
Leia is pretty darned sure that Obi-Wan Kenobi is her only hope.
How the h-e-double-sippy-straws does she know who, or what, Obi-Wan Kenobi is?