Captain Underpants and the Invasion of the Incredibly Naughty Cafeteria Ladies from Outer Space (and the Subsequent Assault of the Equally Evil Lunchroom Zombie Nerds)

 is a book written by Dav Pilkey.

Plot
Three evil aliens, Zorx, Klax, and Jennifer, land on the school roof and plan to take over the Earth by disguising themselves as cafeteria workers and feeding the kids alien food to turn them into evil zombie nerds and then using the kids to take over the world.

Meanwhile, in science class, George and Harold learn about the effects of mixing baking soda and vinegar together (the two ingredients don't mix and make a slimy sticky liquid that fizzes and bubbles). They decide to play a prank on the cafeteria ladies by giving them a recipe that involves mixing baking soda and vinegar. The recipe is disguised as a cupcake recipe, with instructions to make a set of the cupcakes for Mr. Krupp's birthday.

The lunch ladies decide to surprise Mr. Krupp and make cupcakes for the entire school. So instead of mixing a small amount of baking soda and vinegar, they mix a large amount: the result being that the school gets flooded by the slimy green mixture. The lunch ladies get mad at Mr. Krupp because he can't punish George and Harold because there's no proof they did anything wrong and quit.

Zork, Klax, and Jennifer, badly disguised as humans, come in and take the open lunch lady job. Mr. Krupp, despite still having no proof, takes away George and Harold's cafeteria privileges, as punishment for their prank. Meanwhile, the aliens feed Zombie Nerd Juice for lunch, so everyone in the school becomes a zombie nerd. George and Harold learn of the spacemen's plans to feed evil growth juice to the zombie nerds to make them humongous and evil, so it's easier to conquer Earth. George and Harold steal the growth juice and try to get rid of it, but accidentally pour it on a dandelion.

When the boys prove to Mr. Krupp that the lunch ladies are aliens, one of the aliens snaps his tentacle (a physical impossibility) at them, so Mr. Krupp becomes Captain Underpants. They fight the aliens, but are unable to defeat the zombie nerds, so they escape by going on the school roof and into the alien spaceship. Inside they find some anti-zombie nerd juice, some destructive juice and some super-power juice. The aliens come in and lock them in a cell. George and Harold mix up the juices and signs on the ship so the aliens pour destructive juice into the fuel tank, which makes the spaceship explode.

Captain Underpants jumps off the space ship with George and Harold, claiming that he can use tissue paper to swing onto the ground. Of course, the tissue paper cannot support their weight and they fall, using Captain Underpants cape as a parachute (the narrator first joked that they died). They end up landing near the mutated dandilion, which nearly kills Captain Underpants. George and Harold give him super power juice to help him break free.

Mixing anti-zombie nerd juice with root beer and having all the zombies drink it, they turn everyone back to normal. Mr. Krupp is no longer normal, though, because he drank the super-power juice, so now whenever he becomes Captain Underpants, he can fly and use other superpowers, thus making it harder for George and Harold to keep tabs on him.

Comic: Captain Underpants and the Night of the Living Lunch Ladies
The comic in the book starts with the Lunch Ladies cleaning up the cafeteria. The Janitor, without noticing, locks the school up for the weekend. The Lunch Ladies are forced to eat their own food. The Janitor appears after the weekend and finds the Lunch Ladies dead. He takes them up a hill to bury them, but they come back to life as zombies. The Zombie Lunch Ladies attack the school, but Captain Underpants appears and attempts to give them wedgies. However, the Lunch Ladies feel no pain, and a fight begins. Captain Underpants leaps on the top of a building and the Zombie Lunch Ladies follow, but are wrapped up in toilet paper (known as, "The Toilet Paper of Justice). Unfortunately, they pour steak sauce on the toilet paper, and go after Captain Underpants by pushing him off the building, but the Zombie Lunch Ladies also fall off. Captain Underpants uses the rest of the toilet paper to swing to safety. The Zombie Lunch Ladies, however, fall into one trash can each (They die because the trash cans are supposebly filled with thier food). Captain Underpants sings his trademark, "Tra-La-Laaaaa!", as the comic ends.