The Army of the Moons

Europa and Ganymede are leading a growing rebellion of moons against their planetary hosts. While Titan appears to be having second thoughts about the movement, Ganymede believes exposure to other moons' struggles will solidify his commitment to their cause.

Fun Facts:
This episode marks the first time irregular outer moons speak.
This episode is the first to show Jupiter in an annoyed as well as concerned state as he was previously shown to be calm and detached. It is also the first to feature him actually arguing with Saturn.
This is the first episode to mention Planet X.
This is the first episode where Rhea has an official design.