Right as Mickey begins hosting today's show, the guests start crying out to him that they're being victimized by a string of inexplicable thefts occurring that very moment. Ludwig Von Drake then shows up and unveils his newest invention, the Crimeputer (Crime Computer), which he will use to solve the mystery. The machine gives its first clue - that the mystery criminal is "someone in disguise." Goofy starts inquiring all the disguised villains in the House, but this does not help. Von Drake figures that all the villains look guilty, so Mickey has them all caged up. But then the lights briefly go out, and when they come back on, they find that the villains have now been abducted. Von Drake assures the remaining audience members that his Crimeputer will catch the crook before the end of the cartoon they then show.
After the first half of Mickey Foils the Phantom Blot is shown, Mickey asks Minnie, Daisy, and Donald if anything else has gone missing. Goofy reports that plenty of things have been stolen, and also adds that Gus says someone stole all the crabs ("except for Donald", he remarks, much to Donald's umbrage). The Crimeputer then provides the next cryptic clue - that the culprit is "someone who's been here since the beginning." Clarabelle begins spreading rumors over one of the crew members possibly being the culprit, and then the Crimeputer provides another clue - "someone who holds a grudge." Donald, still taking offense at Goofy's "crab" remark, is suspected of being the thief and locked up in the backstage elevator, but then Clarabelle reports to them that Minnie is missing now, thus validating Donald's innocence. The Crimeputer provides the next clue - "someone who is very near" - before the thief then steals Mickey's clothes, along with the rest of the audience.
Following the conclusion of the cartoon, Von Drake, Clarabelle and Daisy are suddenly kidnapped. Mickey reads another clue from the Crimeputer - "the person you least suspect" - which leads Donald and Goofy to believe he's the culprit. They chase Mickey through the club but then suddenly disappear. Mickey begins to think maybe he is the thief, at which point Chief O'Hara comes in and arrests him. But when asked to sign a confession with a leaking pen, Mickey, seeing a familiar-looking ink blot coming from out of the pen, realizes who the culprit is. He opens up the Crimeputer to reveal the thief - the Phantom Blot, who tries to escape in his damaged blimp, only to end up popping it on the spire at the top of the building.
Later, after Mickey has freed everyone including his friends from their imprisonment within the Crimeputer where the Blot had stuffed them, Von Drake announces that he has made an improvement to the machine - it now not only solves crimes, it also catches culprits. Von Drake then remarks that his cartoon that they just showed was so funny that it almost made him die laughing. Chief O'Hara, however, takes that statement seriously and has the Crimeputer drag Von Drake off to jail ("Good thing I've already invented this get-out-of-jail breaker!").
Dastardly deeds for the House of Mouse provided by: "Phantom Blot"
- The 1960's Batman show is referenced when a swirl with Mickey's head (and at one point, Donald's head) spinning is shown.
- This is the only episode of the series to feature a cameo appearance by Percival C. McLeach and Joanna from The Rescuers Down Under.
- Besides Mickey and Chief O'Hara, Max Goof is the only character in this episode who doesn't get kidnapped by the Phantom Blot.
- Clarabelle reports that Minnie was kidnapped, which would indicate that she, along with all the other kidnapped characters, was stuffed inside the Crimeputer. However, after Mickey opens the Crimeputer to free all the trapped characters, Minnie runs in from off-camera, rather than from inside the Crimeputer like the others.
- The Phantom Blot's advertisement at the end says he has been "creating havoc since 1947", but he actually first appeared in 1939.
- Out of all the above-mentioned clues, the "someone who's been here since the beginning" clue could not actually apply to the Phantom Blot since his first appearance was in 1939, sixteen years after Walt Disney started producing cartoons.
- When the guests start accusing one another, Cruella can be seen with a glimpse of her hair entirely white instead half of it being black, no gloves, and the fur coat she's wearing is white as well.