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 techology 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 anihillating 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 after Dexter himself, then take over the city due to the ridicle from his peers. Before he goes to hunt down Dexter, he decides to kick Mandark away, but insteads kicks DeeDee away, who shoves Mandark out of the way. Mandark saves an injured DeeDee and helps her save Dexter, with him 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.