The Challenger

A man is accused of setting his cottage on fire with his wife and child inside. A stranger walks into K.D Phatak's office and confesses that she started the fire in the cottage. She further continues to challenge K.D Phatak to prove her guilty in the court. K.D Phatak decides to take the case up but finds clear evidence for the suspect to be at fault. He further finds out that the suspect was involved in a marital affair with Dimple, who challenged K.D Phatak portraying to be Avni. Who is the real culprit?