This is a story of two sisters, Anjali and Mitali - Anu and Meethi as they are called affectionately at home.

Anjali (Shabana Azmi) and her brother Mahesh were their mother Rewa's (Waheeda Rehman) children from her first marriage to Col. Mathur who had died soon after Mahesh was born. Rewa had brought up her two small children with the help of her late husband's friend - a bachelor by the name of Brigadier Sunil Mathur. Later, Rewa had married Sunil who had been a good surrogate father to Anjali and Mahesh and Mitali (Konkana Sen Sharma) had been born. She was the baby of the family, eighteen years younger than Anjali and thirteen years younger than Mahesh.

Anjali had married quite early but was soon divorced and came back to live with her mother and stepfather. Anjali was brilliant. She was a professor of Physics at a noted university in Calcutta and a writer of repute in her field. Mahesh had become quite successful in the corporate world and lived separately with his wife Padma, their eight year-old twins Samir and Shreya and newborn daughter Shalini. Mitali, the youngest, was the most unfortunate of Rewa's three children.

Whether it was because of Rewa's late pregnancy at the age of forty or simply as a result of some genetic quirk, Mitali or Meethi turned out to be mentally and physically challenged. She suffered both from chronic schizophrenia and epilepsy and had to be cared for by her family. In spite of her illness, Meethi had been quite functional up to her early twenties. She had even got engaged to a young man called Joydeep or Jojo, as she called him. The family, Meethi's elder sister Anu in particular, had been against the engagement at first because of Meethi's medical problems but had finally given in with good grace. There had been a huge engagement ceremony, which was more like a wedding than an engagement.

After that, disaster had struck! Meethi, who was working for a publishing house as a journalist, had gone by herself on a journalistic assignment outside Calcutta where political goons had raped her repeatedly. This incident had acted as a trigger for Meethi's hitherto dormant schizophrenia. Joydeep who was only twenty-six at the time, was unable to relate to Meethi any longer and had broken off the engagement. Meethi had become depressive to the point of being suicidal and had even had to be put away in a home for the mentally ill for a while.

Later she was brought back home but she was never the same again. She now lived completely in a delusional world where she was married to Jojo and was Mrs. Joydeep Ray with five children of her own. She believed that she lived with her husband and children at 15 Park Avenue and would insist on being taken to look for her house, which she could never find.

After Sunil died, Anu became the sole caregiver to her widowed mother and schizophrenic sister. As a result Anu became a much stressed individual and Meethi's doctor Kunal Barua advised her to take a break from work - to go away for a holiday far from her stressful city to a place where the air was pure and the climate was healthy.

Anu, Meethi, their mother Rewa and Meethi's attendant Charu came to Bhutan for the prescribed holiday. By a strange co-incidence Joydeep was here too with his wife Lakshmi and their two children. He saw Meethi quiet unexpectedly while they were sightseeing and felt completely shattered at the change he saw in her. He could not get rid of his feelings of guilt and followed Anu and Meethi to their rented cottage. Anu was stunned to see him and struggled to control the feelings of intense resentment that welled up inside her. She was also worried in case Meethi had a setback when she met him - but strangely, Meethi did not recognize him at all! She continued to call him Mr. Roy after she was introduced to him never dreaming that this was in fact her beloved Jojo.

Meethi became quite friendly with Joydeep who kept coming over to their cottage in spite of his wife's reservations, and confided in him about her husband Jojo and her five children. She entreated him to help her find her house at 15 Park Avenue and to help her get away from her sister whom she imagined to be completely tyrannical!

Joydeep was miserable! He felt like a bigamist! He was in a strange situation where he was happily married to a woman he loved and with whom he had had two lovely children; yet, here was another woman who believed she was married to him and whose world with him was totally real to her! Joydeep was nonplussed. Should he go with her to look for 15 Park Avenue after they returned to Calcutta? They would not be able to find the house, of course - no such house existed except in Meethi's imagination! In fact, there was no Park Avenue on the map of Calcutta! Yet, if he did not go, Joydeep felt, he would be letting Meethi down a second time.

Back in Calcutta Joydeep does go with Meethi to look for 15 Park Avenue - to look for something one can never find. But, is Meethi the only one who is looking for the impossible? Is she the only one with delusions? Aren't we all - the so-called 'sane' people of the world - under delusions of our own?