It starts when Mai tells him to give up. He is twenty-one. It has been five years since his coronation, four years since his father confessed that his mother was banished to, of all places, the Western Air temple, three and a half years since she was traced to the mainland Earth Kingdom. He hasn't found her. Rewards have been announced and posters sent out, the Kyoshi warriors have offered their services and given up, because they are needed elsewhere and there is nothing they can do.
Zuko is hiding it, but he has become obsessed. His mother haunts his dreams and his waking hours, and finally, he breaks down and tells Mai, hoping for sympathy. Mai is impatient. She had expected to be Fire Lady by now, only to be told that Zuko would like it if one member of his immediate family, just one, could see him wed. Mai is fed up, and she tells him to lose hope.
"It's been too long, Zuko," she says. "If she wanted to come back, she would have by now."
Because she really does love him, and she really tries to make him happy, she softens her voice and hesitates before suggesting that possibly his mother is dead.
"Maybe… you should just give up. Move on."
He can't do that, because for eleven years he's had her whispering in his head, that's who you are, someone who keeps fighting even though it's hard, and in the past few years it's been mixed in with you were there for me when I didn't even know I needed you, and I'll never forget that, shining eyes and gratitude and the only person who cried and hugged him closely when she had to leave for home.
He tells Mai, I will never, ever give up on someone I love!
And he can hear her heart break as he walks away, because he has long since given up on trying to make her understand.
He writes to Katara, like he always does, but this time he doesn't ask for advice, he asks her to give him hope. Of all people, she can understand that. He wants a letter full of sympathy and kind words, stories of her mother in the ice and snow, and encouragement.
But Katara knows him better than he knows himself, so she knows it's not words he needs. He needs her to come flying in on Appa two days later at twilight, dressed in black and ready with a tight hug. She whispers in his ear that they'll leave at midnight and find his mother themselves, that she's brought his uncle along to take care of court matters while he's gone, and Zuko is stunned but all he can say is thank you.
They leave at midnight, as planned. Appa is being held outside the city, because the last time he visited he had seen firebenders training, and fallen back into his war-standard methods of roaring scarily and jumping into the middle of the action. They want to avoid that this time, and on the way to the clearing they pass Mai's house.
She is standing on the steps, waiting for them. She doesn't say a word. She walks down the steps and slaps Zuko across the face (because really, she's not that mad or surprised, she just wants him to know that she is not pleased) and goes back inside. Unbelievably (but he believes it), she looks betrayed.
"She… doesn't understand."
Katara puts a hand on his shoulder. "I do."