The first man to notice Edwin Harper was dead remarked only that accountants ought not to sleep beneath filing cabinets. It was an observation made with no expectation of wit, for the cabinet had toppled during the night and pinned the poor fellow beneath its iron weight.

The police surgeon pronounced the affair a most unfortunate accident, and no one thought to dispute him. Harper had worked late, the cabinet had been improperly secured, and fate, which is seldom known for its mercy, had settled the matter.

The accident might have been forgotten by everyone except Harper’s grieving sister had the coroner not emptied the dead man’s pockets. Along with his keys and spectacles was a folded memorandum written in Harper’s careful hand.

It contained only three figures, each followed by a name: Mason, Clarke, and Bell. No explanation accompanied the list.

Harper had been chief auditor for the Ashcroft Trust Company, where fortunes passed through ledgers more quietly than rivers through valleys. It fell to Inspector Lionel Graves to determine why a dead accountant had gone to the trouble of carrying such an odd note.

The answer came before sunset. During a routine inspection prompted by Harper’s unfinished work, the bank discovered that nearly half a million dollars in negotiable bonds had vanished from the secure vault.

There was no broken lock, no smashed window, no sign of violence. The vault required three separate keys, each entrusted to a different officer.

Mason possessed one. Clarke another. Bell the third. The newspapers immediately christened the affair The Problem of Three.

Each officer denied any knowledge of the theft. Their denials were convincing, if only because each appeared equally respectable.

Mason was the elderly cashier whose honesty had become almost legendary. Clarke supervised security and had personally inspected the vault every evening. Bell, young and ambitious, managed investments and possessed every appearance of integrity.

None appeared capable of robbery. Yet without the cooperation of all three, the vault could never have been opened.

Inspector Graves disliked impossible conclusions. Experience had taught him that whenever an investigation presented only one explanation, it was usually the wrong one.

He began not with the vault, but with the dead accountant. Harper’s office revealed stacks of meticulous calculations interrupted by one curious page.

All three columns of numbers ended in identical totals. Graves studied them for nearly an hour before smiling faintly.

The sums balanced too perfectly.

Three separate investment accounts had suffered tiny adjustments over several months. Individually, each discrepancy amounted to almost nothing.

Together, they equaled precisely the value of the missing bonds. Someone had been preparing the robbery for a very long time.

The inspector interviewed Mason first.

The old cashier admitted Harper had questioned several transfers only two days earlier but insisted the differences resulted from clerical corrections. Clarke admitted Harper had requested access logs, and Bell confessed that Harper seemed unusually interested in dormant investment certificates.

Three conversations, three different subjects, and one dead accountant. Graves found the arrangement altogether too neat.

Returning to Harper’s desk, he searched for the access logs Clarke claimed Harper had requested. They were missing, not stolen entirely, as only one page had been removed.

That single absence interested Graves more than every complete ledger in the building.

He visited the vault. The locks bore no scratches.

Dust lay undisturbed around every hinge except beneath a small ventilation grate near the ceiling. There he found faint gray powder.

Concrete dust. Someone had recently loosened the grate, but not to enter the vault.

The realization came almost immediately. To watch it.

Harper had suspected irregularities. Rather than accuse respected officers without proof, he had quietly observed them.

Hidden above the vault through the ventilation passage, he had watched whoever entered, but before reporting his discovery, he had returned to his office. There, working late beneath overloaded filing cabinets, he had met with extraordinary bad luck.

The cabinet collapsed, the witness died, and the robbery continued. The theft had not been exposed by brilliant planning, nor by the thief’s mistake, but by chance itself.

Graves gathered the three officers together. “Gentlemen,” he said, “only one of you is guilty.”

Bell laughed softly. “The newspapers would disagree.”

“The newspapers are frequently mistaken.”

He turned toward Clarke. “You removed one page from the access register because it showed you entered the vault alone after business hours.”

Clarke remained silent.

“You knew everyone would assume all three keys were necessary. They were not.”

Bell frowned.

Graves continued.

“You discovered months ago that routine maintenance left the emergency locking bolts disengaged for nearly fifteen minutes after inspection. During that interval, one authorized key was sufficient. You concealed the maintenance reports, altered the log, and waited until Harper died. His accident convinced you no one remained to expose the weakness.”

Mason stared at Clarke in disbelief. “But my key—”

“Was never used,” Graves interrupted.

“Nor mine,” Bell whispered.

Clarke’s shoulders sagged. “I didn’t kill Harper.”

“I know,” said Graves quietly. “That is the one honest statement you’ve made.”

For several moments, no one spoke.

Finally, Clarke nodded. “He frightened me. He kept asking questions. Then he died before I could act. I thought fortune had chosen my side.”

“No,” Graves replied. “Fortune merely delayed justice.”

Outside, reporters waited eagerly for tales of secret conspiracies involving three trusted officers and an ingenious robbery requiring perfect cooperation.

Instead, they learned something far stranger. There had never been three thieves, only three names, three keys, and three assumptions.

The greatest obstacle to solving the mystery had not been the cleverness of the robber, but the willingness of everyone to believe that because the number three appeared everywhere, it must explain everything. Inspector Graves knew better.

Numbers, like people, often concealed the truth behind appearances. And in this curious affair, it was not the problem of three that solved the mystery.

It was the mistake of believing there had ever been three problems at all.