You'd think eleven-year old rich kid, Marcus Fleming, has it made, but he doesn't. He often feels like a disappointment, struggling to live up to his dad's high standards.

One day, Marcus holds open a door for a very sickly-looking man. The thanks he gets is a sneeze that covers him in thick green slime. Marcus soon discovers that this slime has given him the ability to shoot radioactive mucus out of his mouth and nose. After fine-tuning his abilities, he dubs himself the superhero, Mucus Phlegmball, but is determined to keep his powers a secret.

When mutant squirrels threaten the town, Marcus feels compelled to step up, but worries he can't handle it alone. He reveals his secret to his best friend, Chunk, and to Dr B., the sickly scientist who sneezed on him, and they agree to work together.

Through radioactive snot-filled time traveling adventures, Marcus is continually challenged and faces many of his fears. Can this insecure young man summon the courage to handle the responsibility of being a super hero?