The main character is Sandy, played by Olivia Newton John. Some may think John Travolta, as Danny Zuko, owns this throughline. He has the star power and acts as the story's protagonist, however, the white keds we skip in belong to Sandy. She's "no stranger to heartbreak" and neither are we. Sandy is concerned with the progress she is making in a "fish out of water" situation--a virginal Aussie transplanted to raunchy Rydell in her senior year of high school. As her influence character, Danny maintains a fixed attitude--he has a black leather jacket with the collar up image to uphold and must keep his tender impulses in check. 
But now he's on view among his friends -- the male gang called the T-Birds, which hangs out with the Pink Ladies -- and has his cool and independent rep to maintain. The Pink Ladies ridicule Sandy's virginal ways, especially tough-talking Betty Rizzo (Channing). A rival gang, the Scorpions, wants to race the top T Birds car, and a national TV dance show is coming to town. And of course, graduation looms. Sandy basically has to decide whether to change her ways to keep her man. 
Grease is a musical about some American high school kids in the 1950's. It ripped off Romeo and Juliet. For Karma, High School Musical ripped Grease off.