The truth behind Zuko's maternal great grandfather was revealed in 'The Avatar and the Fire Lord'.

It also sets up the main arc of the series, the search for Ursa.

This chapter is the first in a series of episodes in Book 4 designed to wrap up some open-ended parts of the Avatar series with the revelation that Zuko is the great grandson of Roku as well as Sozin. Sokka declares that Team Avatar will go with Aang and Zuko to look for Ursa. Upon hearing this, Aang states that he will help Zuko find his mother when he's ready to head out, which Sokka agrees with and volunteers the rest of the team to go as well, including Mai who doesn't want to be left behind again. Prompted by these words, Zuko confesses to the others that he's Roku's great grandson. The scroll reveals how Ta Min was pregnant very late in life and gave birth to a son, Kuzon, Aang's old Fire Nation friend. Zuko takes the scroll and reads it allowed.

In the attic of the house, Zuko finds a scroll labeled, 'The Last Testament of Ta Min'. They find it over grown with ivy, which they tear off. While relaxing at Ember Island a short time after the end of Book 3, Zuko informs Aang about the house Ta Min bought on Ember Island, and the team decides to take a look at it.

Watch avatar the last airbender book 3 ep4