"I think Star was so sad that no one wanted to celebrate Feast Day that she crated a personality for this disgusting mass as a coping mechanism."
— Robin sharing his theory with Silkie regarding Parry.
"Parasite" is the seventeenth episode of Season 1 of Teen Titans Go!, and the seventeenth episode overall. It first aired on 16 July 2013 on Cartoon Network.
Starfire acquires a space parasite, and insists on treating it like a pet. The parasite succeeds in winning over all of the Titans except Robin, who alone sees the growing organism as the threat it is.
Starfire, in space, is collecting food when she is stung by a plant, causing her to grow a boil on her side. She vaporizes the plant as a result and returns to Titans Tower where Robin is lecturing Beast Boy, Cyborg, and Raven for not putting away video games after they finish playing them. As soon as he spots Starfire, Robin forgets about the lecture and asks about her whereabouts. The Tamaranian reveals that she was gathering ingredients to cook for a holiday celebrated by her people and asks the Titans if they would join her in celebrating it. They reluctantly agree and Cyborg offers to cook the food for her, but Starfire reveals she has prepared an assortment of Tamaranean cuisine consisting of glurk scales, fried fish legs, and mouth worms. Disturbed by the food, Cyborg and Beast Boy lie that they need to celebrate their respective baby cousins' birthdays who lives near the movie theater and disembark. Starfire offers a mouth worm to Raven, but she retreats to her room through a portal. Robin, in an attempt to impress Starfire, offers to eat one, but the mouth worm screams when he attempts to bite it. He activates a smoke grenade and flees from Starfire, who sadly calls out for him.
In her room, Starfire tells Silkie that she's happy the silkworm is always there for her and offers him a mouth worm, but he scurries away. She wonders if there was anyone who'd want to participate in her people's traditions and scratches her bump before attempting to fall asleep. However, the boil begins to glow and Starfire is excited that it offered to participate in Feast Day. The next day, Cyborg asks the Titans if he can pull off skinny jeans as Starfire prepares more food for the holiday. She turns to greet them, and her boil is met with disgust by the Titans. She explains that the parasite is named Parry and he's offered to join her in celebrating Feast Day. Robin declares the parasite to be a threat, but Starfire reassures him that he's harmless. The leader, however, protests that their encounter with the parasite would result in the Titans fighting a giant space spider on the other side of the galaxy. Starfire tells Robin that Parry thinks he's being rude. Cyborg attempts to explain that Parry can't talk, but Starfire claims that they communicate through her mind. Raven labels this as not normal which Starfire says is a title she is accused of by her friends as well before flying away.
In his room, Robin tells Silkie that Starfire is crazy for talking to a parasite. The silkworm attempts to leave, but Robin takes him to his computer where he decides to show him something. He accidentally clicks on a link revealing pictures of him in a speedo and hastily clicks through all of them before bringing up a chart. He explains that he believes Starfire gave the parasite a personality due to being sad over the fact that none of her friends wanted to celebrate Feast Day with her. He rushes to tell the other Titans of his experiment, but finds that they have become friends with Parry as the five of them share a joke. Parry begins to throb and Starfire mentions that she's already asked them and they declined. When questioned by Cyborg, Starfire reveals that Parry asked if the Titans would join them for Feast Day and the Titans all change their mind and agree to participate. Starfire asks Robin if he would participate as well and he agrees in order to keep an eye on Parry. Starfire disembarks with Parry in order to show him around Jump City before Feast Day and Robin decides to follow them to make sure Parry doesn't hurt Starfire.
As the Titans set up for Feast Day, Robin tells Raven that Parry didn't try anything during his and Starfire's tour of the city and labels him more patient and fiendish than he thought. Raven suggests that Robin should consider the possibility of Parry being a nice parasite while he's a creepy guy in the bushes, but Robin waves the accusation aside claiming once again that they'll end up fighting a spider on the other side of the galaxy. The Titans gather for Feast Day with Starfire translating a toast from Parry that thanks the Titans for welcoming him into their home. Robin interrupts and makes a toast of his own, telling the Titans that they have to remove Parry from Starfire before it kills her. He further explains that the parasite's personality is only a figment of Starfire's imagination and that he has to go, but Starfire protests the action due to her and Parry being best friends. Robin mocks Starfire by claiming that, through her reasoning, everyone should have a parasite friend and reveals a jar filled with them that he brought for the occasion. He douses himself with the leeches and they begin sucking his life force, causing him to grow skinnier and weaker. Hurt, Starfire reveals that Parry is the only thing that makes her feel normal around the tower and leaves in tears. Beast Boy chides Robin, but the severely drained leader asks for someone to call a doctor.
That night, Robin, dressed in a bio-hazard suit, stealthily enters Starfire's room and prepares to saw off Parry in Starfire's sleep. He activates the machine, causing Starfire to wake up and question his actions. Robin prepares to slice off the parasite, but as he grabs the boil, he is transported to the inside of his mind. There, he meets Parry, causing Robin to realize that Starfire was telling the truth the other time. Parry wonders if he feels like a jerk, and Robin apologizes by claiming he should have made an effort to understand him, but Parry claims it isn't him he should try to understand. Robin realizes he means Starfire and calls the parasite all right. In order to show their new found friendship, Parry creates a cape for Robin's cape and Robin returns to Starfire's room. Starfire wonders if Robin talked to Parry and he reveals that he did before apologizing to Starfire for calling her traditions strange as he sometimes behaves weirdly. Starfire reassures that his weird behavior is more than sometimes when Parry communicates with her, making her sad.
On the roof of Titans Tower, the Titans gather to bid farewell to Parry as he detaches himself to leave. Robin begins to cry due to Parry's departure, but the parasite transforms into a Cironielian Chrysalis Eater and activates a portal that transports the Titans into the other side of the galaxy. Robin expresses his delight to be right through a song when the Cironielian Chrysalis Eater crushes the Titans.