Impulse is the fourth book of the Morphers.
When two youths wreck havoc in the city with their powers, Caleb must muster his strength to defeat them, even with Rachel's concern.
The story starts off with the Morphers practicing with their new found powers, but fail to shape shift into any creature after several attempts. Afterwards, Caleb decides to go home, causing Rachel to tag along with him on the way, much to the former's annoyance. Caleb asks why she doing it, causing the latter to reply that she always noticed Caleb constantly being withdrawn, unlike the others.
Suddenly, the two are alerted by a smoke, not far from the neighborhood, causing them to arrive at the scene, along with the others, who demorph from their osprey forms. It's revealed the perpetrator of the attack is from two teens, who both have extreme agility and super strength. The two reveal themselves as Jeremiah and Ezra and manages to gain the upper hand with their new found powers with ease by causing a shop to collapse unto the Morphers. However, Caleb manages to get out of the debris, and pins Ezra. Before he could attack the latter, Rachel stops him, telling him it's not right to kill him, even if he is pure evil. Rachel asks Ezra, why he's doing this, to which he replies that he wanted to feel excitement. Rachel points out that his actions have only caused more fear, but Ezra just runs off with Jeremiah with his powers.
Later, Rueben decides to split the group up into order to find the duo with Rachel and Caleb going north, while he, Nick, and Noah head south. After walking for a while, Rachel notices that Caleb is no longer with her, and decides to find him. After much searching, she finds him, training by himself. Rachel asks him why he's doing this, but Caleb replies to her stating he needs enough strength and know their weak point in order to stop the duo. Rachel retorts, stating that strength is not going to stop them, but by words and tells him that he lacks control, as evident from earlier. At first, Caleb is unfazed, but eventually agrees to follow her advice, after some persuading.
Jeremiah and Ezra head to another part of the city and decides to cause more mayhem. However, Ezra is having doubts about Rachel's words from earlier, but Jeremiah convinces him to forget about it. Rachel and Caleb then witness lots of people running, alerting them to the duo's presence. The two youths continue unleashing their powers, until Rachel and Caleb arrive to stop them. Caleb tries to convince Ezra to stop, causing Ezra to reveal that he just wanted to have fun. Caleb doesn't buy it, and tells him that without control, his life will be destructive, causing Rachel's words to come back at him. The two Morphers tries to fight the two teens, but are still no match for them. Suddenly, a man in black arrives knocks the two Morphers unconscious, and tells them that he can help train their powers. The two youths are hesitant at first, stating that their power could harm people, but the man tells to follow him to secret place.
The man in black brings the two to an empty room, and tells them to test their powers. The two then have fun, causing as much wreckage as possible. Afterwards, the man in black congratulates the two by stating that they have passed their test. He tells them to strap themselves to a bed. However, the Morphers have tracked the duo to their hideout and demands that they confess. Jeremiah and Ezra reveals that they were best friends since elementary school and were mentally gifted, but both of their parents died when they were at a young age. The two are then raised in a facility, where about a few months earlier, were approached by a man who promised to help fulfill their potential. The man in black states that he is the one who mutated the two and granted then superpowers and using them as soldiers to destroy the Morphers He then reveals his true form, a Vulture Organism and immediately attacks the Morphers.
The Morphers are having a hard time fighting the Organism, being able to dodge all of their attacks. Jeremiah and Ezra tries to attack at top of the building, causing debris to collapse on Caleb, however, he is pushed away by Rachel in the nick of time who in turn gets knocked out by the debris. Caleb, enraged, decides to attack but is quickly beaten down by Jeremiah. Ezra, realising his actions, decides to fight back. He tries to attack his former friend, but is eventually. Just as Jeremiah is about land the killing blow, Ezra asks him if he is still having fun and shows him the Morphers fighting the Vulture Organism, causing him to realise his mistakes. The two teens then lament that they have wasted their lives on dangerous acts and that their powers can only cause harm. Ezra, assures Jeremiah, who after listening to Rachel's words, decides to use their gifts to help others.
Meanwhile, the other three Morphers are knocked down by the Organism. However, Jeremiah and Ezra reveals the Organism's weak point, which is located at the chest. The Morphers, after listening to the duo's words, manages to defeat the Organism by attacking him in the chest, along with Jeremiah and Ezra's help. Afterwards, Caleb manages to drag Rachel out out the debris, and tries to revive her, but is unable to. After several tries, she manages to come about. The two teens thanks the Morphers, especially Rachel and Caleb for their help and promises to surrender themselves to the police for their crimes.
A week later, the Morphers visit Jeremiah and Ezra in a juvenile prison. The duo reveals that they now mastered the art of control and is currently improving for the time being. Caleb then thanks Rachel for helping him open out and the promises to be her friend from now on.