The third book in the Secret Coders series from Gene Luen Yang and Mike Holmes.
Stately Academy is no ordinary school: it was once home to an elite institute where teachers, students, and robots worked together to unravel the mysteries of coding. Hopper, Eni, and Josh won't rest until they've learned the whole story, but they aren't the only ones interested in the school's past. Principal Dean is hot on their trail, demanding that the coders

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Volume 3 in the mind-bending new series from Gene Luen Yang! Hopper never dreamed that there was a connection between the Bee School and her missing father -- but it turns out that Albert Gracie was also a student of this mysterious coding school. His disappearance can only be the work of Dr. One-Zero. Once a star pupil of the Bee School, Dr. One-Zero is now a green-skinned villain on the hunt for "the most powerful turtle in the world." And the coders...

Dr. One-Zero won't stop until the whole town--no, the whole world--embraces the "true happiness" found in his poisonous potion, Green Pop. And now that he has the Turtle of Light, he's virtually unstoppable. There's one weapon that can defeat him: another Turtle of Light. Unfortunately, they can only be found in another dimension! To open a portal to this new world, Hopper, Eni, and Josh's coding skills will be put to the test.