Regardless of whether or not STAR WARS was planned as a standalone movie, it was presented as an homage to the multi-chapter cliffhanger serials like FLASH GORDON, plunging you into the middle of a story already in progress.
Adults in 1977 who grew up with those films picked up on the nostalgia (& often remarked about it in reviews), but most of us kids had no idea! The title crawl "story recap" is the first clue. 🎬😎👍