Donnie Finds a Relic is a back up story printed in Teenage Mutant Ninja Turtles: Amazing Adventures issue #4. It apparently takes place after Karai's Vendetta and before Showdown, Part 1 during the events of the TV series' first season—a time when April O'Neil was living in the turtles' lair to hide from both the Kraang Hive Mind and the Foot Clan.
The four turtles are out at a junkyard. Leonardo and Donatello are using their respective brothers Michelangelo and Raphael as improvised toboggans, sliding down slopes of garbage with Mikey and Raph's hard carapaces against the surface. Afterward, Donnie notices something interesting lodged inside one of the crevices of Raph's shell. To his surprise, it's a piece of technology he has never seen before—a data cassette with information stored on tape wound onto two spools. He does not recognize it as an audio cassette, never having needed to use one before.
Back in the lair, Donnie tries in vain to see if the tape will fit into the cavities of any common appliances—the VHS VCR, the kitchen toaster, and so forth. April walks in to find that Donnie has already started building an improvised elaborate device in the hopes of finding a way to playback the cassette's data. April looks over Donnie's shoulder, the moment of physical closeness causing him to blush. Donnie says the cassette is like himself—there should be something special hiding inside its shell, and he is determined to found out what it is. Meanwhile, Leo and Raph are irritated by Donnie's obsession with the device, but Donnie remains fixated on trying to read the cassette's data. April, however, already recognizes the audio cassette for what it is, telling Donnie all he needs is something like a boom box. Donnie has never heard of a "boom box", and doubts he could find one anyway, and laments that he might not be able to find out what priceless knowledge could be on the tape. April suddenly has an idea.
April comes out onto the surface, having snuck past Splinter and wearing a purple hoodie to try to remain discreet. April understands that as smart as Donnie is, he isn't very street-smart, and that what he wants can probably be found at a pawn shop. April finds such a pawn shop, but her discretion has failed—she has been noticed by some Foot Soldiers.
Inside the pawn shop, it isn't long before April finds a boom box. Unfortunately, it costs more money than she can afford. The clerk says he accepts cash, credit or exchange. April has already thought to bring along some junk in her blue backpack. The clerk rejects most of it as garbage, but spots a record needle and accepts it as an exchange.
After April leaves the pawn shop, she is ambushed by three Foot Ninja. Improvising her still-new kunoichi skills, she quickly pulls off her hoodie and swings it at one of the ninja. She also grabs two garbage can lids and strikes two ninja with them. April throws her backpack into the hands of the third ninja, which distracts him enough so she can jump and flip over him, kicking him from behind. April reclaims the backpack and makes her escape, annoyed that her favorite hoodie has been ruined in the process.
Back in the lair, Donnie's improvised cassette playback machine has grown monstrous in size and complexity, filling much of the room. Even so, he has not been successful getting the cassette to play, and thinks he may have to start over from scratch. But April arrives, takes the boom box out of her backpack and gives it to Donnie, reassuring him that she didn't have to steal it.
The tape turned out to be demo tape by an obscure late 1980s amateur rock band called Coco and the Pebbles. They never became famous or successful and have faded into history, but the turtles quickly take a liking to the demo tape's music. As the music plays and they are eating pizza together, Mikey reveals he has improvised pepperoni and pineapple from a pizza as fake eyes and teeth. April reacts by giving Mikey a friendly noogie, and the gathering of April and turtles quickly erupts into a food fight. The story ends with Raph's back to the viewer as he reaches for pizza, revealing a still-unnoticed floppy disk lodged in one of the crevices of his shell.