Hum Kaun Hai looks at the relationship between Frank James Williams (Amitabh Bachchan) and his wife Sandra (Dimple Kapadia). Since Frank is an army Major, he went to war some time back and there's no sign of his return. As time passes, his wife slowly loses hope that he will come back. To relieve her pain, Sandra focuses her love towards their two children, and although she's a strict disciplinarian, she raises them utmost care.
She receives some help from the family's servants, Martha Pinto (Nanny), Edgar (Gardener), and Maria (Helper). Together, they live in an isolated brick mansion set amidst thick woods and mist. When eerie and spooky occurrences begin within the household, Sandra decides to consult the local priest. On the way, she runs into her husband, who is in a dazed state and seems to be delusional.
The children are obviously thrilled to see their father again, however when the daughter reveals a haunting secret, Frank enlists the help of his friend Virendra (Dharmendra) to get to the bottom of the spooky activities surrounding the house. Frank with his friend Virendra takes it upon themselves to get to the bottom of the mystery and intrigue, surrounding the Williams.