The magically musical duo of Goopy Gyne and Bagha Byne make a comeback in this sequel, where they are invited to the court of the Hirak Raja (The Diamond King), for their musical skills. They are to perform at the kingdom's Jubilee Celebrations.
Goopy and Bagha are bored with their lives as crown princes of Shundi and Halla. They are looking for a change, which comes in the form of a chance to visit Hirak Rajya (Land of Diamonds), known for its huge diamond mines. They jocundly set out for Hirak Rajya. But little do they know of the machinations of Hirok Raja (Utpal Dutt). The king is a tyrant. Diamonds and riches get pent up in his treasuries, while his subjects starve and suffer. Those who protest are taken care of in the 'Jantarmantar', a chamber for brainwashing devised by the scientist (Santosh Dutta), whose full name is Gobeshok Gobochondro Gyanotirtho Gyanorotno Gyanambudhi Gyanochuramoni. His ministers are mere puppets. The only enemy the king has in his land is Udayan Pandit (Soumitra Chatterjee). He is a school teacher and, more than that, he is a believer of values. The king forcefully closes his school down. Udayan flees to hide in the mountains.
Meanwhile, Goopy and Bagha are on their way to Hirok Rajya. By coincidence, they meet Udayan, who intimates them of the king's true nature. The two impress Udayan with their magical powers, who plans to use them against the tyrant. Goopy and Bagha agree. The duo then head into Hirok Rajya, where they are welcomed with grandeur. They entertain the rogue king,fooling him into believing that they think he is great. They rob the treasury (which was guarded by a tiger) using their migical music, for bribing the guards.
The king has his tricks, too. He captures Udayan and all his students, and takes them to the Jantarmantar for brainwashing. But Goopy Bagha have already reached there using their magical powers. They have also bribed the Gobeshok onto their side, with the guards. On reaching the laboratory, the king and his ministers are stunned magically by Goopy's singing and then pushed into the Brainwashing machine.
