Khanna, a wealthy man, hires an assassin to murder his rival, Sohanlal. He the victim's niece Asha of his death.

Asha wins a trip to an island with six others: Barrister Rakesh, Dharamdas, Kishan, Dr. Acharya, Madhusudan Sharma and Kitty Kelly. They travel there by plane, which is forced to make an emergency landing. It takes off, stranding the passengers with crew member Anand. He claims to know nothing about events. They hear a woman singing a song.

They proceed to the mansion, where Dharamdas finds a diary. Anand reads its contents to everyone. It reveals they are all connected to a crime and are responsible for it. The writer says he will kill all of them.

Everybody retires to their rooms, Anand spies on them. He discovers Dr. Acharya has a bottle of poison and Dharamdas has a revolver. Kishan catches Anand spying but does nothing. The butler acts suspiciously, proving there is at least one more person in the house.

The next day, Kishan spots a ship and fails sending a distress signal. Anand starts flirting with Asha, and things heat up between Rakesh and Kitty. Everyone is suspicious of everybody. Rakesh's prime suspect is Anand. Anand and Asha find Kishan's corpse. His killer left a note saying Kishan murdered Sohanlal. The group deduces Dharamdas killed Kishan. He pleads innocence. Sharma asks for one day to prove his innocence. Dharamdas is found dead by Anand and Asha. Anand concludes the culprit is within the group. Rakesh believes the doctor poisoned Dharamdas.

It becomes clear everyone in the house was connected to Sohanlal. Kitty is Sohanlal's secretary. Rakesh wrote Sohanlal's will on Khanna's orders. Kitty sent the will to Rakesh on Khanna’s instructions, though neither knew about the other. Asha suspects Anand, who says nothing when she asks about himself. Anand spots Rakesh hiding an axe. Dr. Acharya arrives screaming that Sharma has been killed by an axe. The killer left another note stating Sharma was Khanna's co-conspirator in Sohanlal's murder.

Anand accuses Rakesh of Sharma's murder. Rakesh is now unsure of Anand's guilt. Dr. Acharya catches the butler acting suspiciously. He learns the butler's secret and theyscuffle. The doctor enters the dining room, utters Anand's name, and collapses before Asha and Kitty. They realise he has been stabbed. Rakesh believes Anand is the killer. He denies it. Asha now questioning her faith in Anand, drinks with Kitty.

Rakesh advises Asha not to go anywhere alone. They continue looking Kitty who has gone for a swim, but senses she is not alone. She flees, but the killer corners her and hangs her. Rakesh and Asha arrive. Rakesh's suspicions about Anand are confirmed after spotting his hat near Kitty's corpse. Rakesh sees Anand, starts chasing him but loses his trail. Outraged, Rakesh returns to the mansion and tries to rape Asha. She escapes then runs into Rakesh as he collapses with two daggers in his chest. The mansion's lights go out; the killer has arrived. Asha is next.

The killer approaches her, she faints. He carries her into a secret room and revives her. Sharma, the killer, tells Asha he convinced Dr. Acharya to help him fake his own death. He then murdered the doctor. Anand appears and reveals he is a police inspector. Sharma is an escaped convict named Madanlal.

Madanlal overpowers Anand. Asha ties them up and recounts his story. He, Khanna, and Sohanlal were smuggling partners. When Madanlal was caught, they convinced him to take the blame. After Madanlal was convicted and jailed, Sohanlal raped his sister. She committed suicide. Sohanlal and Khanna killed Madanlal's brother. Khanna had Sohanlal killed to usurp his money. Everyone Madanlal killed was involved in the plan to murder Sohanlal. The "ghost" singing the ominous song is the butler's mentally ill sister. Madanlal killed Khanna and ensured his targets "won" the lucky draw.

Madanlal plays Russian Roulette with Anand and Asha. The butler frees Anand. When it becomes obvious Asha will be shot, Anand attacks Madanlal. In the ensuing melee, Madanlal escapes the mansion and runs towards the shore. A plane full of policemen arrives and Madanlal is arrested. Anand, Asha, the butler and his sister leave in the plane.

• Nanda as Asha
• Manoj Kumar as Anand/ Inspector Anand
• Pran as Barrister Rakesh
• Helen as Kitty Kelly
• Mehmood as the butler
• Dhumal as Dharamdas
• Madan Puri as Dr. Acharya
• Tarun Bose as Madhusudan Sharma/ Madanlal
• Manmohan as Kishan
• Laxmi Chhaya as the masked dancer

Critic Shahid Khan rated the soundtrack 9 out of 10 stating, "Gumnaam is sometimes unfairly overlooked but I believe that it is one of Shankar-Jaikishan's best albums."

According to film expert Rajesh Subramanian, a cold war prevailed between Mehmood and Manoj Kumar during the making of the film. Kumar tried to convince the director to discard the song "Hum kaale hain toh kya hua", which has picturised on Mehmood, from the film. However, it was kept and went on to become a hit and one of the highlights of the film. An English-language version of the song titled "The She I Love", sung by Mohammed Rafi, was also recorded.

Box office

Gumnaam became a box office hit. It was the 8th highest-grossing film in India in 1965, grossing ?2.6 crore.^[4] This was equivalent to $5.46 million in 1965,^{[n 1]} and is equivalent to US$42 million or ?275 crore^[6] in 2016.

Awards and nominations

• Filmfare Nomination for Best Supporting Actress – Helen
• Filmfare Nomination for Best Supporting Actor – Mehmood
• Filmfare Best Art Direction Award – Colour Movie – S.S. Samel^[7]