The Original Trilogy/Rebellion Years.
A single-minded determination kept the little droid on mission: deliver the Death Star plans to Obi-Wan Kenobi on Tatooine. He did just that, and later proved crucial in finding Princess Leia aboard the Death Star, and taking a laser blast for the team during Luke’s trench run.
Again Artoo comes to the rescue in Episode V. A quick conversation with Cloud City’s main computer tells him that the Falcon’s hyperdrive is offline; he fixes it, allowing Luke, Leia, Chewie and Lando to escape. In Return of the Jedi, he even serves as an undercover agent, infiltrating Jabba’s palace. Later, he’d launch Luke’s Lightsaber into the Jedi’s hand to effectuate Han’s rescue. Han, of course, goes on to lead the successful mission to deactivate the Death Star’s shield generator.