The Clone Wars movie, not an episode per se.
Wikipedia offers a synopsis:
With the help of Obi-Wan Kenobi and Anakin Skywalker, the clones steadily advance on the Separatists' forces, gaining the Republic an early victory. It doesn't last long, though, as the droid army soon returns. With no communications or the ability to fly in reinforcements, the fate of the few remaining clone soldiers are in the hands of Obi-Wan and Anakin.
A shuttle arrives, but without the needed reinforcements. Instead, it is a young Padawan named Ahsoka Tano, who insists that she has been sent by Master Yoda to serve under Anakin. The battle soon commences yet again, with the Separatist forces advancing behind an expanding energy shield that artillery can't penetrate. Anakin and Ahsoka succeed in penetrating the enemy lines while Obi-Wan stalls for time by holding a fake surrender negotiation with the droid army commander.