Secret mission to Tatooine

"Thank you, Artoo-Detoo. You were my only hope."

- Leia Organa, to R2-D2

Search on Tatooine
R2 found Obi-Wan Kenobi.

Aftermath
"I'm sorry, Artoo. We did the right thing. And you were wonderful―you found Ben Kenobi, you brought the plans here. That gave us a chance. I shouldn't be complaining. You were perfect."

- Leia Organa, to R2-D2

The plans were ultimately delivered to Yavin 4, allowing the Battle of Yavin to take place.