As Byomkesh and Ajit walked back through the rain-slicked streets, Ajit marveled. “How did you know, Byomkesh?” Byomkesh lit a cigarette, exhaling slowly. “The thread was the key, Ajit. It told me someone had been under the bed, waiting. The rest was human nature—greed, fear, and a mother’s love. Truth, like a river, always finds its way.”

In the bustling heart of 1930s Calcutta, where the Hooghly River shimmered under a monsoon sky, Detective Byomkesh Bakshi leaned back in his armchair, his keen eyes fixed on the nervous figure of Mrs. Sukanya Mitra. The widow, draped in a white sari, clutched a silk handkerchief, her face a canvas of distress. Byomkesh’s friend and chronicler, Ajit Bandyopadhyay, sat nearby, scribbling notes in his battered notebook, while the ceiling fan whirred lazily overhead.

“Mr. Bakshi, it’s gone,” Mrs. Mitra whispered, her voice trembling. “The Mitra family heirloom—a gold locket with an emerald the size of a pigeon’s egg—has vanished. It’s not just a jewel; it’s a symbol of our lineage. If it’s not found, my family’s honor will be tarnished forever.”

Byomkesh’s brow furrowed. “Tell me everything, Mrs. When did you last see it?”

Mrs. Mitra explained that the locket was kept in a locked rosewood box in her bedroom safe. Last night, during a family gathering at her ancestral home in North Calcutta, she’d shown it to her relatives. Her late husband’s brother, Anil Mitra; his wife, Kamini; and their son, Debashish; plus her own daughter, Leela, were present. After the guests left, she placed the locket back in the safe, locked it, and went to bed. This morning, the box was empty.

“Any signs of tampering?” Byomkesh asked, his fingers steepled rhythmically on the armrest.

“None,” she replied. “The safe was locked, the key was with me. The windows were latched, the doors bolted. It’s as if the locket vanished into thin air.”

Byomkesh exchanged a glance with Ajit, who raised an eyebrow. “We’ll visit your home,” Byomkesh said. “I need to see the scene.”

The Mitra residence was a grand, if slightly faded, mansion with sprawling verandahs and intricate jali work. Byomkesh inspected the bedroom: a heavy teak bed, a dressing table cluttered with silver combs, and the safe—a sturdy Chubb model—embedded in the wall. He examined the safe’s lock with a magnifying glass. “No scratches, no forced entry,” he murmured. The windows, too, were intact, their iron bars unbent.

He questioned the household. Anil Mitra, a portly man with a booming voice, claimed he’d left early to attend a club meeting. Kamini, sharp-eyed and tight-lipped, said she’d been busy managing the kitchen. Debashish, a lanky youth with a nervous tic, swore he’d spent the evening playing cards with Leela. Leela herself, a quiet girl with soulful eyes, confirmed this, adding that she’d retired to her room after the guests departed.

Byomkesh paced the drawing room, his mind a whirl of deduction. “Ajit,” he said softly, “this is no ordinary theft. The thief knew the house, the safe, and the routine. Yet, no one could have entered that room without a key.”

Ajit frowned. “Could it be an inside job? But who? Everyone has an alibi.”

Byomkesh’s eyes gleamed. “Alibis are like shadows, Ajit—they shift with the light. Let’s test them.”

He summoned the family again, this time observing their reactions closely. When he mentioned the locket’s value—not just monetary, but sentimental—Kamini’s fingers tightened on her dupatta. Debashish’s tic worsened. Anil blustered about family pride, while Leela stared at the floor.

Byomkesh then asked Mrs. Mitra to recreate the previous evening. She showed how she’d opened the safe, displayed the locket, and locked it again. As she mimicked placing the key under her pillow, Byomkesh’s gaze darted to the bed. He knelt, peering beneath it, and retrieved a tiny sliver of green thread caught on a nail.

“Interesting,” he said, holding it to the light. “This isn’t from your sari, Mrs. Mitra. Nor from Leela’s. It’s silk, likely from a shawl.”

Kamini paled. “I don’t own a green shawl,” she said quickly.

Byomkesh smiled thinly. “We’ll see.” He turned to Debashish. “You play cards, yes? Show me your deck.”

Debashish hesitated, then produced a worn pack. Byomkesh shuffled through it, pausing at the ace of spades. “Curious,” he said. “This card is slightly bent, as if pressed hard. Perhaps to signal something?”

Debashish stammered, “I—I don’t know what you mean.”

Byomkesh’s voice hardened. “I think you do. Last night, you and Leela played cards, but it was a distraction. While Mrs. Mitra was occupied, someone slipped into her room. The key was taken from under her pillow, the safe opened, and the locket removed. The thief wore a green shawl—Kamini’s shawl, which you, Debashish, borrowed to disguise yourself.”

Kamini gasped. “This is outrageous!”

Byomkesh ignored her. “You, Debashish, needed money. Your gambling debts are no secret. You planned this with your mother’s help. She provided the shawl, and you used the card game to keep Leela and Mrs. Mitra busy. The bent ace was your signal to Kamini that the key was in hand.”

He turned to Mrs. Mitra. “Search Kamini’s room. You’ll find the shawl. And check Debashish’s lodgings for the locket.”

Anil roared, “Enough! I won’t have my family slandered!”

But Mrs. Mitra, steel in her eyes, nodded. The search was swift. In Kamini’s wardrobe, a green silk shawl was found, missing a thread. At Debashish’s rented room, the locket gleamed inside a cigarette tin.

Debashish broke down, confessing. “I needed the money… I was desperate. Mother only helped to protect me.”

Kamini wept, her defiance crumbling. Mrs. Mitra, torn between fury and pity, agreed not to press charges if the locket was returned and Debashish left Calcutta.

As Byomkesh and Ajit walked back through the rain-slicked streets, Ajit marveled. “How did you know, Byomkesh?” Byomkesh lit a cigarette, exhaling slowly. “The thread was the key, Ajit. It told me someone had been under the bed, waiting. The rest was human nature—greed, fear, and a mother’s love. Truth, like a river, always finds its way.”

And so, another mystery unraveled in the city of secrets, with Byomkesh Bakshi’s mind as sharp as the monsoon’s edge.