Byomkesh Bakshi was visiting his friend Ganpat Negi. He found out that his friend wife had just killed a burglar in self defence.
Bakshi asks Ganpat about what happened and he told that "His wife were watching television when suddenly the bell rang. She thought that it is her husband Ganpat but she found the burglar who attacked her instantly and she got so frightened that she killed the burglar immediately with the knife.
Byomkesh Bakshi asked the police to arrest here friend wife.
Why ?

Ganpat wife was planning to kill him and she planned to kill him as soon as he opened the door else why would she had the knife in her hand especially watching television.