The Grand-Daddy of All Inventions is the 9th episode of season 3, and the 55th episode of Dexter's Laboratory overall that aired on November 30, 2001. In this episode, Dexter takes a trip to see his grandfather. While there, he learns that his grandfather, despite his outward boring appearance, has a laboratory of his own.
Dexter's mom takes him to his grandpa's house. At first he thinks it's going to be a boring day until he discovers that his grandpa has a secret lab too! Now he's super excited, but soon he starts missing his parents and sister.
Dexter is working on an invention in his laboratory which is an energy processor that can make Infinite Energy or Free Energy a supposedly scientifically impossibility energy that never runs out and can be used for the world. Everything is going according to plans until there is a power outage and his experiment gets ruined. He tries to do it again but then his mom calls him down to go to his grandpa's house.
When Dexter is at his Grandpa's house, he is very bored with what he does. Then, his grandpa tells him that he has a secret laboratory too. He goes over there with him and it turns out just to be the garage. Grandpa shows him his latest invention which is just a bunch of random stuff. Dexter leaves because he is bored with it and his mom takes him home.
After he leaves, grandpa's invention turns out to be a machine that makes Infinite Energy or 'Free Energy' source just like what Dexter was trying to build back in his lab back at home.
Grandpa's house has the exact same design as Dexter's house.
This segment was part of the final episode to have Christine Cavanaugh as the voice of Dexter before her retirement from voice acting (though her very last performance as Dexter would be in "Tele-Trauma").
Although this episode along with "Copping an Aptitude" and "A Failed Lab Experiment" were aired on November 30, 2001, they were produced in the same year according to the credits. They are also the final aired episode aired in 2001.