A Boy and His Sister: A Dexter's Laboratory Movie

A Boy and His Sister (marketed as A Boy and His Sister: A Dexter's Laboratory Movie) is a 2026 American computer-animated adventure comedy film directed by Genndy Tartakovsky (creator of the original series) and written by series writers Paul Rudish, Amy Keating Rogers, Michael Ryan, and Walt Dohrn. Candi Milo, Kat Cressida, Eddie Deezen, Jeff Bennett and Kath Soucie reprise their roles from the series; accompanied by Mckenna Grace, Peter Capaldi, Charlize Theron, Jim Parsons, and Dan Aykroyd. This is the first computer-animated feature film from Cartoon Network Studios, with co-production by Warner Animation Group.

A Boy and His Sister had its premiere in Hollywood on March 25, 2026, and was released theatrically in the United States for the series' 30th anniversary on April 10, by Warner Bros. Pictures. It grossed $150.2 million on a $90 million budget. The film was met with positive reviews, with critics praising the voice cast, humor, and heart, while the choice of computer-animation was mixed.

Plot
Dexter, having graduated from Huber Elementary and moving to Huber University, is about to receive a Nobel prize for his invention that ends world hunger; an invention he had been working on in secrecy due to his childish older sister, DeeDee, who had ruined several of his inventions from the start. DeeDee, eavesdropping on Dexter's pursuits, was heartbroken but determined to sabotage his dreams. As the night of the ceremony had begun, DeeDee had stopped the machine by spitting a wad a gum she was chewing during Mandark's showcase of his visual Mind-Screen invention. After Dexter's world hunger machine had made a good first impression, the machine spits out various amounts of food at the audience. Dexter fortunately stops the machine in time, but discovers the wad of gum in the gears which costs him the Nobel prize. In retaliation and realization that DeeDee was responsible, Dexter blames her and starts a food fights with her.

After the ceremony, Dexter was disappointed and upset about his dream being crushed until he gets a call from the committee that one of their security cameras caught footage of DeeDee spitting her gum into the gears as everyone was oblivious; exposing DeeDee for sabotaging his machine and reinstating the Nobel prize to him. As DeeDee was exposed, she ends up grounded for it. Dexter later enters her bedroom to take pity on her, whereas DeeDee angrily explains that she hates the idea of him going to college and never got the childhood with him; Dexter apologizes for it, but he cannot pass up his second opportunity to start anew as a college student. The next day, the family visits Huber University, and Dexter is amazed by its facilities and classes to his liking. After the tour, he meets the Dean's daughter Marie and bonds with her over her interests in technology and video game developing, with his parents proud that he expands his social life.

Meanwhile, Mandark, walking the streets of Genius Grove, plans on going with his plan of annihilating Dexter; his former substitute teacher Professor Ian Adams helps him in his pursuit due to his hatred of Dexter as well. With DeeDee, she and her parents are called to the elementary school and given word that DeeDee is failing her classes and will be held back a grade if this continues, leaving her in utter dismay; suggesting she be tutored to help her get back on track, Dexter was chosen as an easy choice. During her tutoring, Dexter had thought about DeeDee's words last night and decides to make it up to her by spending time and playing with her before his move to college next week. As the week was over, DeeDee thanks him for bonding with her, with Dexter showing satisfaction from it.

As the family drives to the university, a laser, which was shot by Mandark and Adams in their mech, had almost hit them. Noticing Mandark in, Dexter addresses the mech his parents and they try to allude it through the city. Before they could go any farther, Adams shoots another laser which slices the family car in half, stopping them in their tracks. As Mandark was about to shoot Dexter, DeeDee tosses throwable debris at the mech to distract them long enough for Dexter to escape and for their parents to go to the police. Irritated, Adams prepares to dispose of DeeDee, but Mandark intervenes out of affections towards her, which has Adams eject him from the mech for his weakness and eliminate Dexter himself, then take over the city due to the ridicule from his peers. Before he goes to hunt down Dexter, he decides to kick Mandark away, which ends up with DeeDee ending up kicked after she shoves Mandark out of the way. Mandark saves an injured DeeDee and helps her save Dexter, as well as reluctantly promising her to stop his rivalry with him in return.

Dexter runs to the university and takes refuse with the students and employees who have heard about the mech on the news. Mandark and DeeDee enter Dexter's lab to retrieve some of his equipment to rescue him, as well as using the Robo-Dexo 2000 for transport and safety. Adams finds Dexter at the university from the help of a newspaper headline of the latter's pursuits; DeeDee arrives at the last minute and faces off with Adams while Mandark retrieves Dexter and sabotages Adams' mech by setting off the self-destruct feature he had installed. As Dexter was about to take over, DeeDee ejects him out as she wants him safe and to make up for her actions. Mandark hides under Adams' mech and sets off the self-destruct feature via voice activation, with DeeDee tossing the mech into the sky before it would explode. Dexter forgives DeeDee for her recklessness and thanks her for rescuing him, along with choosing not to go to the university for the sake of spending the rest of his childhood with her, much to her delight. As the police arrive after their parents were told to contact, Adams had survived the explosion and plummets on one of the police car roofs before being arrested.

As time goes on, Dexter had begun a respecting bond DeeDee after forgiving each other; expanded his social life from befriending Marie and forging a friendship with Mandark after the latter had ended their rivalry; and returned to Huber Elementary, working as a substitute teacher. DeeDee had also begun a relationship with Mandark.

In a post-credits scene, DeeDee enters Dexter's lab to call him for dinner. While searching for him, she finds and presses a button. As Dexter was exiting the bathroom, she finds DeeDee covered in ashes telling him how she realizes how he felt when she destroyed his inventions, thus receiving a taste of her own medicine.

Voice cast

 * Kat Cressida as DeeDee, Dexter's oblivious but well-meaning sister.
 * Candi Milo as Dexter, DeeDee's intellectual younger brother.
 * Eddie Deezen as Mandark Astronomov, Dexter's arch-rival with an infatuation towards DeeDee.
 * Jeff Bennett as:
 * Dexter and DeeDee's father
 * Windbear, Mandark's father.
 * Kath Soucie as:
 * Dexter and DeeDee's mother
 * Mee Mee, one of DeeDee's friends.
 * Oceanbird, Mandark's mother.
 * Peter Capaldi as Professor Ian Adams, a former subsititude teacher in Dexter's class who was ridiculed by the scientific community for his bogus findings. His name is a play on the word "ion atoms."
 * Jim Parsons as Dean Julius Dean, the dean of Huber University.
 * Mckenna Grace as Marie Dean, Jullius' video game-obsessed daughter who develops a quick bond with Dexter on electronics.
 * Charlize Theron as Denise Dean, Jullius' wife and Marie's mother.
 * Dan Aykroyd as Gary Zeh, the host of the Nobel Prize award ceremony.
 * Kimberly Brooks as LeeLee, one of DeeDee's friends.