Although Derek didn't appear much in the first season, his origins were shown in the episode "The Substitute", where Milo Murphy and his friends accidentally created a sentient blob that attacked the students and two time-travelling agents named Vinnie Dakota and Balthazar Cavendish. In the end, the blob left the school and came into contact with a broken pistachio sapling on the lawn, causing it to be replenished to its former glory and resulting the birth of King Pistachion, who would breed his own army of Pistachions (including Derek) to take over the world.
In the hour-long special "Missing Milo", Derek appeared briefly when King Pistachion ordered him and several other Pistachions to capture Balthazar Cavendish, Vinnie Dakota, and Milo Murphy to prevent them from going back in time to destroy the sapling. However, the trio manage to escape in their time-travelling limo, but Derek has managed to catch up by holding onto the limo. As they travel through the time stream, Vinnie notices Derek and used a bag of clocks to knock him off the limo, leaving a helpless Derek to stumble around the time stream.
Following the deaths of King Pistachion and his other Pistaction back in Milo's time, Derek ends in the year 1955. Smiling at this, Derek happily runs off as he intends to get revenge on the heroes for defeating his father and wiping out his race from existence.
Fungus Among Us
In the episode Fungus Among Us, it is revealed that Derek spend the next 10 years secretly plotting his own revenge against humankind for wiping out his race. Disguising himself as an intern for the Dr. Zone show in 1965, Derek uses the show's profit to formulate a successful plot that involves extinguishing the human race and replacing them with new Pistachions in disguise. He even was able to create his own army of Pistachions to diguise themselves as several of the show's crewmembers to help him out. This causes the time stream to change, allowing the Pistachions to round up the majority of the population.
With only Milo, Diogee, Dakota, Cavendish & Malthson remaining as the only ones to evade the Pistachions' capture, they seek to find the world's first time-traveler Professor Time to get some answers to stop Derek and the Pistachions from taking over the world. However, in the episode's cliffhanger ending, the five end up meeting the evil scientist Heinz Doofenshmirtz (who too has avoided being imprisoned by the Pistachions for unknown reasons) after being trapped in his lair by accident.
- It is unknown what happened to Derek and the Pistachions following the cliffhanger of Fungus Among Us, but it is confirmed that the rest of the story will be told in the upcoming Milo Murphy's Law/Phineas and Ferb crossover that will air in 2018.