The Beast Within is the 35th episode of the Teen Titans series and the ninth in the third season.
Inside an animal testing laboratory, the Teen Titans fight Adonis. The battlesuited villain easily defeats four of the Titans, but enrages Beast Boy to the point where the latter literally cuts loose and attacks Adonis furiously. During their fight, they puncture a tank containing strange chemicals and get drenched by them. After Adonis is beaten, Beast Boy exits the lab in an extremely aggressive mood, leaving the other Titans bewildered.The next morning, Beast Boy begins to display very untypical behavior, even for him: He suddenly develops an appetite for meat, becomes downright rude and aggressive, and under stress even turns into his animal forms more frequently. When he finally feels unjustly provoked by Raven, he attempts to attack her, but is stopped and rebuked by the others, who are put off by his manners of late. In a rage, Beast Boy retreats to his quarters, smashing up his furniture, and suddenly transforms into a huge hulking monster. Raven, passing by his room, hears a strange sound, but as she attempts to see what is going on, a monster comes out and attacks her. Upon her fearful cry, Robin, Starfire and Cyborg rally, but they find Beast Boy and Raven missing and Beast Boy's quarters devastated by claw marks. Following Raven's tracer, they find her in the sewers hanging unconscious and injured from the fangs of a big green werewolf monster. The creature proves a very tough opponent to fight, and only with the greatest effort do the Titans manage to subdue it - to find out to their immense startlement that the creature was none other than Beast Boy!
In the Tower's sickbay, Cyborg notices upon analysis that a strange genetic compound has mingled with Beast Boy's DNA. Distraught by Raven's condition and the fear that he is the one responsible, and stressed by Robin's forceful questioning, Beast Boy transforms into the werebeast yet again and immediately goes after Raven. The Titans drive him away and back into the city; Robin and Cyborg take up pursuit, while Starfire stays behind to take care of Raven. Some time later, Raven comes to, but she testifies that Beast Boy had not attacked, but saved her.Robin and Cyborg follow Beast Boy back into the sewers - to encounter actually two werebeasts brutally fighting each other. After a long and ardorous struggle, Beast Boy manages to subdue his opponent, who turns back into his true self upon losing consciousness: Adonis. After the strange compound is identified as the cause and an antidote administered, Beast Boy sits on the Tower island's shore, where he is joined by Raven. While Beast Boy is still fearful about this animalistic self lurking deep inside him, Raven gently assures him that it is this bestial side which makes him all the more human. Feeling more mature now, Beast Boy considers a change of name, spoiling the tender moment.
- It's a likely possibility that in the sickbay/infirmary, Beast Boy (as the Beast) was in a rush to check up on Raven (which the others misinterpreted as intent to harm/attack). When he sniffed her and a look of anger crossed his face, there's a chance he looked angry because he knew she was hurt and he got angry at Adonis. That may be the reason he escaped the infirmary: to go after Adonis.
- Beast Boy: I think my appendix and my spleen just switched places.
- Adonis: You wimps think you can take down Adonis? Bring it on.
Beast Boy: Dude, it is totally brunged!...branged!
Raven: Ooh, bad grammar. That oughta scare him.
- Adonis: What's the matter, wisp? Don't you know how to fight like a man?
Beast Boy: I'm not a man ... I'm an animal!
- Raven: If it wasn't for that beast, I might not be here right now. Having that thing inside doesn't make you an animal. Knowing when to let it out is what makes you a man.
Beast Boy: Hmm. Maybe you should call me...Beast Man from now on.
Raven: We're having a moment here. Don't ruin it.
Beast Boy: Beast Dude? (Raven sighs)
- Starfire: She was in your teeth.
Beast Boy: That's a lie!
- Cyborg: (while punching Adonis) Come on, big man! Show me what you've got! Show me what you've got! (Adonis grabs him) Well, ok then. (he gets thrown to a wall)
- Adonis: Nobody out muscles Adonis!
- Adonis: (Purring to Raven) Your a fiesty one