When Luke hears R2D2's pre-recorded message from Princess Leia, he clocks that she's looking for Obi-Wan Kenobi. He doesn't know an Obi-Wan, but he does know a Ben Kenobi, who he calls a "strange, old hermit," and he brings R2D2 and C3PO to meet him. Turns out, Ben is Obi-Wan's secret identity.
But why does Luke know Ben? Other movies have established that Uncle Owen and Aunt Beru know that Obi-Wan is a Jedi Master, he was friends with Luke's father, and that they need to keep Luke hidden from Darth Vader. But Uncle Owen and Obi-Wan don't seem to get along, and Uncle Owen calls him a "crazy old man" when Luke brings him up. Under what circumstances has Luke met Ben before?