The Green Ninja Ninjago: Masters of Spinjitzu : Season 1 Episode 10

The Ninja are suspicious when Lord Garmadon, brought by Sensei Wu from the realm of darkness he fled to, moves in to help searching for his missing son - but realize they have to work together in their attempt to rescue Lloyd from the Serpentine while they dig for the third Fang Blade in the heart of the Fire Temple. Kai, after being held back by his ambition to become the Green Ninja, reaches his true potential and rescues Lloyd. It is also revealed, though Kai's own realization, that Lloyd is the destined Green Ninja. This causes grief to both Wu and Garmadon, since their family has now been further divided.