The Evil Smell Switcher is the mysterious and main antagonist of Mercer Mayer's book "Little Monster Private Eye: The Smelly Mystery." He is the one who changes the smells and switches them from where they belong to anywhere else. At the night in which Little Monster and his male friends are sleeping over at Yallapappus Manor (Yally's house), the next morning, the Evil Smell-Switcher switches smells. For starters, he switches the smells of the peanut butter and the orange juice, makes a salad smell like perfume, makes flowers smell like fish, fish smell like candy, and garbage smell good, and even switches more smells.

According to the computer game adaptation, when Little Monster and his trusty assistant Detective Kerploppus find certain clues, he rounds everybody else up at Yallapappus Manor and reveals which of the boys at the sleepover is the evil smell switcher. Of course, because like the other boys, he was there at the time, Little Monster was one of the six suspects, but being the good guy (well, one of them if you count Det. Kerploppus), in no gameplay is he ever the perpetrator. For one thing, he could not have been in bed taking a nap and made the sound of leaving the note on his bedroom door at the same time. It is either Little Laff, Icky, Jerry Bombanat, Errg, or Yally. When the player selects the correct suspect, based on the clues found, the culprit tries to get away, only to trip on a cord that then starts setting a trap.

After the culprit is trapped, Little Monster questions the reason for the smell-switching.

  • If the culprit is Little Laff, he says it was just a joke. Little Monster tells him it wasn't funny and that crime never is.
  • If it was Icky, he said he was doing it for the danger, as if danger was his middle name. Little Monster corrects him saying his (Iggy's) middle name is Horace, and that changing names is a dangerous game.
  • Jerry Bombanat? He did it as a scientific experiment. Little Monster tells him that the next time he does a scientific experiment, he should do it in his lab and leave Monsterville's citizens' noses out of it.
  • If it's Errg, it's because (as he says at first) "the cheese made [him] do it." Actually, when Little Monster asks, "Come again?", he says it again and that he was trying to make everything smell like his favorite food: cheese. Little Monster tells him that if he does it again, he will have plenty of time to age like an old cheese, and he'll do that in jail.
  • If Yally is the culprit, he said it was because he saw it in a spy movie. Little Monster tells him that life is not a spy movie, and that he should know that art imitates life, but life doesn't imitate art.

After either reason is explained to either culprit, Detective Kerploppus agrees with him. The Smell-Switcher asks Little Monster and his assistant, "You're not gonna/going to call the cops, are you?" "No, even worse," says Little Monster. "We're going to call your mother." Then the switcher screams "no" about it.

It is never known how either Evil Smell Switcher is punished by his mother.