"The Beast Within" is the thirty-fifth episode of the Teen Titans series and nineth episode of Season 3.
After a battle with Adonis, Beast Boy has a dramatic change where he is rude, eats meat and has a major temper that could burst at any moment. This causes the Titans to worry but soon things become worse when Raven and Beast Boy go missing and find Raven in the jaws of a werewolf-like beast. The creature is soon revealed to be Beast Boy and the Titans believe he is the one who attacked Raven. Will the team be forced to send Beast Boy to prison or is there something else lurking in the shadows?
Inside an animal testing laboratory, the Teen Titans fight Adonis. The battle-suited 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, telling the other Titans that he got sick of being pushed around, 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, meaner, and both more aggressive and hostile, and under stress even turns into his animal forms more frequently. When he finally feels unjustly provoked by Raven, he attempts to fight her, but is stopped and rebuked by the others, who are put off by his manners as of late. Beast Boy walks away to his quarters, destroying his furniture while ranting, 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 hearing Raven's 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 to be 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 surprise that the creature was none other than Beast Boy; dazed, confused, and unaware of where he is.
In the Tower's sickbay, Cyborg notices upon analysis that a strange genetic compound has mingled with Beast Boy's DNA. Cyborg explains that because of Beast Boy morphing abilities, his genetic code has always been unstable. He assumes that Beast Boy may be finally falling apart. Robin starts to interrogate Beast Boy about what happened, but he firmly states that he cannot remember. When Robin starts to push Beast Boy too hard, the latter looses control and once again transforms into the werebeast, 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 guard Raven. Robin ends up shooting an exploding Birdarang at the werebeast, which causes it to cry out loudly. Some time later, Raven wakes up, 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 fighting each other. Initially, the other werebeast overwhelms Beast Boy, violently beating and throwing him about the sewers while taking no damage in return. After smashing him through the floor and into a lower level of the sewers, the other werebeast begins mercilessly beating the incapacitated Beast Boy until he appears to be unconscious. As the foe turns away, Beast Boy quickly recovers and brutally beats the beast in revenge, eventually knocking it out. Beast Boy then turns back into his true self, as does the opposing unconscious werebeast, who is revealed to be Adonis, who had broken into the tower to attack Beast Boy. Robin and Cyborg then meet in their location, and just then realize just how powerful their supposed "weakest" member of the Teen Titans was.
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. Raven explains to Beast Boy what happened the night of the attack: Adonis had broken in to the Tower, with intention to attack him and soon attacked her as well. Beast Boy is also disturbed and disappointed of his recent behavior and apologizes to Raven, but she assures him that it wasn't his fault as he was not himself. Beast Boy, however is also still shaken by the knowledge of what this new form is capable of, and is still fearful about this animalistic self lurking deep inside him. Raven gently reassures him that knowing when to let this bestial side out makes him all the more human. Feeling more mature now, Beast Boy considers a change of name. Raven tells him that they're having a moment, and to not ruin it.
- 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 became angry at Adonis. That may be the reason he escaped the infirmary: to go after Adonis and get revenge on him.
- The animals that Beast Boy turned into in this episode are a wolf, a bull, a baboon, a tiger, a Bigfoot/Sasquatch, a rhinoceros, an Ankylosaurus, a lion, a bear, and the Werebeast.
- Beast Boy was annoyed with Raven, because she makes fun of him, insults him, and tells him his sentence is grammatically incorrect (while he was facing Adonis).
- We've learned that Starfire's favorite show is "World of Fungus".
- Beast Boy ate meat and eggs for the first and only time.
- Beast Boy's natural hands, feet and arms are seen for the first time due to the rips in his costume (not including the sight gags in "Forces of Nature" and "Transformation").
- If you listen to a certain part of the dialogue closely, it sounds like Beast Boy is saying "RAVEN!" in a muffled voice as the werebeast. This is the first time he is able to speak as an animal, yet the second time he spoke while not in human form.
- If you listen to Raven's scream closely, it sounds more like it came from Starfire rather than Raven. Some people believe that Raven and Starfire have the same voice actor, yet this is proven untrue.
- When Raven is extending her powers over Adonis' body to stop him cold and when she screams "Let me go," she gains a lower jaw fang similar to that of Beast Boy's.
- This is the second time Beast Boy's toy cymbal monkey appeared in this episode from ""Fear Itself", but it was later seen when Beast Boy threw it against the wall thus breaking it.
- Beast Boy still used his Werebeast form in "The End - Part 1".
- When Robin grabs a piece of Raven's cloak off the gate, it is torn. However, afterwards, Raven's cloak is shown to be undamaged.
- Raven's position on the ground changes after Beast Boy (as the Werebeast) puts her on the floor. At first, she was on her front side, but when Starfire flies over to her, she is on her back.
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