Disclaimer: I don't own anything. Except Zak.
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Zim sat in his seat, ignoring Ms. Bitters' doom rant as usual. The Tallest had contacted him earlier that day, and told him about Zak coming to train him. They had said that despite his mission, he was not technically encoded as an Invader anymore, and that if he accepted (and passed) Zak's training, he would be pardoned of all his crimes and his Invader class would be restored. They had cut the transmission before Zim had had a chance to respond. Zim was still thinking about what they had said, and the more he thought about it, the more it made sense. He was an Invader, but the encounter with Sizz-Lor had reminded him that the Tallest had forgotten to change his ranking back after he quit his banishment. The Tallest just wanted him to prove his mettle before officially changing his rank back to Invader.
As Zim reassured himself of these things, Ms. Bitters had gotten on the phone with the principal and was now complaining angrily. Zim was snapped out of his thoughts by Ms. Bitters slamming down the phone. "Class!" she shouted angrily, "We have a new student joining us today."
At that moment the door slammed open and in marched what appeared to be a normal human. He wore a red shirt with black sleeves, and black pants. His hair was black and unkempt. His eyes, though human, were piercing and red. He turned to look at the class before his gaze stopped on Zim. "I am Zak." He stated calmly. "I just got into town. My, eh, family moves around quite a bit so I won't be here long. But while I'm here…you'd better make sure you stay out of my way" he finished dangerously.
"All right Zak, since you won't be here long you can have Willy's old seat." She pointed to the desk in the back of the room that had belonged to Willy before he had become the student president. Zak marched to the back, dropping a small piece of paper on Zim's desk as he passed.
Zim snatched the paper. It was a note: Meet me behind the building after skool lets out. 'So, he's the one the Tallest spoke of,' Zim thought, and turned to look back at Zak.
At lunch that day, Dib was bothering Zim as usual. This time, Dib had taken to pelting Zim with the cafeteria food, which had taken to eating part of Zim's wig. Dib was about to launch the mashed potatoes when a lunch tray flew out of nowhere and beaned Dib right in the back of his giant head, knocking him forward into the plate of mashed potatoes, which immediately proceeded to start devouring his nose right off his face. Gaz snickered at his pain and Zim could be seen laughing hysterically. Also laughing, although nobody could see it, was Zak.
After skool let out, Zim snuck around back, making absolutely sure Dib wasn't following him. Zak was already there. "Zim, I'm sure you know why I'm here. The Tallest sent me here to train you. We will begin immediately. Take me to your base." Zim was a bit surprised at Zak's straightforward approach, and was about to make a comment when Zak snapped at him to get moving, so he decided that it would be better to speak with Zak in the safety of his base anyway.
Zak was less than impressed with Zim's base, but made no comment as Zim led him up the walk. Unbeknownst to Zim, Dib was watching from the bushes.
"What is Zak doing with Zim?" Dib asked himself quietly "Zak just got here, so the only way is if-" Dib was suddenly cut off. Zak had launched a grappling hook (hidden in his cyber-exoskeleton) from the back of his wrist, which had grabbed on to Dib's coat. With a snap of his wrist, Zak sent Dib flying. Zim could only watch, astounded, as Dib came crashing down a few yards outside Zim's gate. Zak laughed briefly before following Zim into the house.
Once down in the lab, Zim removed his contacts and wig, and Zak's human form faded away to reveal his true self. "Okay, Zak," said Zim, "What is this all about?"
"Like I said, the Tallest sent me here to train you." Zak explained calmly.
"And how did you do that thing with Dib?" Zim demanded.
"My antennae have been greatly enhanced, just like the rest of my body. I have all sorts of weapons integrated into the cyber-exoskeleton I wear. Plus my sensory organs are far more sensitive than a normal Irken's. I could hear that large-headed kid talking to himself so I tossed him away. But enough with the introductions. Your training begins now."