Torn from his How to be a Detective book by his boss, Keaton sweeps out a movie theater. After a break to gaze into a sweet shop and a wrestling match with a sticky piece of paper, he copes with a series of people searching for lost money in the trash pile. Keaton finds none for himself, so he settles for buying a dollar box of candy (the price is easily changed to $4.00) and he pays a call on McGuire. The awkward lovers are interrupted by Crane, who, fresh from stealing and pawning the girl’s father’s watch, presents her with a much bigger box of candy. Father, accompanied by the handyman, discovers that his watch is gone and Keaton takes charge of the case. He starts by searching everybody, but when he himself is searched, the $4.00 pawn ticket is found (placed in his pocket by his rival) and the girl’s father tells him never to come back. He leaves. Undaunted as a detective, he shadows the villian closely to a train platform, where Crane tricks him into a refrigerator car. Keaton escapes through the top and rides a water pipe to the ground, which promptly douses him.
The wet detective goes back to his projectionist job, starting the feature Hearts and Pearls. It involves the theft of a pearl necklace. Meanwhile, McGuire asks the pawnbroker for a description of the man who left the watch and learns the identity of the true thief. Keaton falls asleep as the film unspools; while he dreams the people in his life replace the characters. He joins them onscreen. The film takes him to the house’s front door, a garden, a street, a cliff top, a lion’s den, in front of a train in a desert, the ocean, a snowy forest, back to the garden, and then fades out.
Later, the father discovers that his pearls are missing. He phones the world’s greatest detective, Sherlock Jr. In preparation, the two thieves plan booby traps: and explosive pool ball and a hatchet over a chair. The crime-crushing criminologist arrives, surveys the suspects and the scene, and neatly evades a poisoned drink and the traps.
The next day Sherlock Jr. follows Crane into a building. Tricked into being trapped on the roof, he rides a railroad crossing guard arm down into Crane’s back seat. Crane drives to his hideout and goes in. Sherlock Jr.’s assistant Gillette (ever-ready in a bad scrape) hops off of the back bumper and they examine what he brought along: a woman’s dress packed in a covered hoop. They place it in the window as the criminals look at the pearls. Keaton whistles on the porch and the gang drags him inside, where he learns that the girl was kidnapped. He grabs the pearls and dives out the window into the old lady costume. He meanders away, but one thug notices and gives chase. Joined by another, they corner Keaton in a dead end, but Gillette is there, prepared: Keaton disappears into the display case he’s wearing around his neck. Gillette goes away with a touch of indigestion and the criminals find a revolving panel. Keaton rotates them and latches it. Another gangster chases him and a motorcycle cop pulls him over for speeding. It’s Gillette in disguise, and Keaton hops onto the handlebars. Soon after they set off, Gillette falls off and driverless, Keaton rides through intersections, past dirt-slinging ditch diggers, into a stag party tug-of-war, over an incomplete bridge (two trucks fill in the gap at the last moment), through a downed tree (workers blow it up right before he hits it), beneath a truck, and in front of a speeding train. When he notices Gillette’s absence he crashes, flying into the house where McGuire is held. The gang pulls up and Keaton and McGuire steal their car. After a chase, Keaton vanquishes them with the exploding pool ball. He stops the car but the four wheel breaks halt only the undercarriage; the body scoots into a lake. He returns the pearls to McGuire and the car sinks.
Back in the projection booth Keaton wakes up. McGuire comes in to say they’ve made a terrible mistake. After some clumsiness, Keaton looks to the film on the screen for instruction. He imitates the hand-holding, kisses, and ring giving he sees. However, he doesn't know what to make of the twin babies who populate the final shot. — Lisle Foote