The Adventures of Captain Underpants

The Adventures of Captain Underpants is an American children's novel by Dav Pilkey, the first novel in the "Captain Underpants" series. It was published on September 1, 1997, and became a hit with children across America.
Plot summary.
One day, fourth graders George and Harold pull a series of practical jokes at the school's football game, from putting black pepper in the cheerleaders' pom-poms to filling the ball with helium, which causes the school to forfeit. The next day, their principal Mr. Krupp reveals that he now has a videotape of them preparing their pranks. Mr. Krupp blackmails George and Harold: unless they obey him, he will give copies of the tape to the football team. The boys wake up at 6 AM to wash his car and mow the grass for Mr. Krupp, not smile at all during school time and do extra homework. In order to escape this, George orders a "3-D Hypno-Ring" from the Li’l Wiseguy Novelty Company which they receive after 4–6 weeks of grueling labor. Purposefully arriving late the next day so they would be sent to the office, the boys hypnotize Mr. Krupp and Harold replaces the video with one of his little sister's "Boomer the Purple Dragon" sing-along videos. 
The two begin taking turns making Mr. Krupp act in ridiculous ways, such as behaving like a chicken, a monkey, and then finally into the superhero from their own comics, Captain Underpants. However, Mr. Krupp jumps out the window to fight crime and the boys grab some supplies such as fake dog feces. "Captain Underpants" eventually confronts two bank robbers at Frank's bank, who fall in hysterics. The police are about to arrest Captain Underpants too, before the boys whisk Captain Underpants away. Before they can change their principal back, the trio witnesses two robots stealing a large crystal and Captain Underpants' cape gets caught on their van, dragging him (and the boys) to an old, abandoned warehouse. There, the trio meet the evil Dr. Diaper, who plans to use the crystal to power his Laser-Matic 2000 to blow up the Moon, destroying every major city on Earth [which happens when Tippy Tinklertrousers changes what happens in book nine] so that he can take over the planet. George and Harold escape and hide, but Captain Underpants is tied up.
George slingshots fake doggy doo-doo between Dr. Diaper's feet and he departs to change his diaper, while the boys destroy the robots and free Captain Underpants. Harold pulls the machine's self-destruct lever, thinking it's an "off" switch. Dr. Diaper returns and, enraged, aims his Diaper-Matic 2000 ray gun, but only that before Captain Underpants shoots underwear at Dr. Diaper's face. (It's later revealed shown that it was his own underwear - he comes out wearing a barrel soon after.) The four manage to escape the warehouse before it explodes and they then leave the doctor tied up and gagged outside the police station with a note that says "arrest me". The three return to the school, but they've lost the ring's manual. In desperation, George dumps a vase of water on Mr. Krupp's head and the principal resolves to give the video to the football team. George finds the manual, but throws it away, but it warns about one going back into the trance and back to normal. Meanwhile, the Knuckleheads love the video and decide to rename themselves the Purple Dragon Sing-a-Long Friends, even though the change doesn't go very well with the fans. George and Harold resume their comic book making and prank-playing. However, they have to keep an eye on Mr. Krupp, because for an unknown reason (obviously due to the warning on the back of the manual ordering ring users not to drench a hypnotized person over the head with water, or it will turn them back and forth from reality to trance if they hear fingers snapping), Mr. Krupp turns back into Captain Underpants every time someone snaps their fingers around him.
Title change.
Early printings of the book have "An Epic Novel by Dav Pilkey" instead of "The First Epic Novel by Dav Pilkey", and the "Little Apple" logo on the cover.
Development.
On why he focused on underwear in the book, Pilkey stated "I think underwear is funny because you’re not supposed to laugh at it."