THE ACCIDENTAL PRIME MINISTER is the story of the ex-Prime Minister of India. In 2004, the United Progressive Alliance (UPA), which has Indian National Congress as the principle party, wins the Lok Sabha elections. Sonia Gandhi (Suzanne Bernert), president of Congress and chairperson of UPA, is all set to become the Prime Minister. But the opposition protests against this development since she's not a natural born citizen of India.
Hence Sonia decides to find someone capable and who also enjoys the confidence of the allies, to take up the post of Prime Minister. After careful deliberation, she asks Dr. Manmohan Singh (Anupam Kher) to be India's next PM. Dr. Singh does so and soon realises that all the major decisions would be taken by Sonia Gandhi herself and her subordinate, Ahmed Patel (Vipin Sharma). Dr. Manmohan Singh meanwhile asks journalist Sanjaya Baru (Akshaye Khanna) to be his media advisor. Sanjaya knows Dr. Singh since more than a decade and has utmost respect for him.
He tries his best to strengthen his image in the media and continuously tells him to make himself heard. Dr. Manmohan Singh however is too soft spoken and has no qualms in letting others take credit for his work. He also agrees to be remote controlled by Sonia Gandhi and her son Rahul Gandhi (Arjun Mathur), the latter poised to be India's next Prime Minister. What happens next forms the rest of the film.