Would you like to reach Nirvana? Then wander along to admire some beautiful court paintings that are about to go on show at the British Museum. This week the museum launches its Indian Summer season,encompassing anything from evening lectures through a themed restaurant menu to a collaboration with Kew Gardens in which the the forecourt will be transformed into a celebration of sub-continental fecundity. But at the heart of the festivities are the exotic visions of the Garden and Cosmos exhibition. This show brings to Britain, for the first time, an exceptional loan of more than 50 paintings produced for the ruling maharajahs of a vast northwest Indian kingdom, once known as Marwar, now the state of Rajasthan.

The show looks at one little facet of sub-continental culture: the paintings produced between 1725 and 1843 for a dynastic succession of three rulers of the Rathore clan, which presided over the arid but magnificent wildernesses of their desert empire from the great forts of Mehrangarh in Jodhpur (the home of these paintings) and Ahhichatragarn in Nagaur. The Rathore rulers consolidated their power by making marital alliances with the incoming Mughals. This led to a new stability and a consequent flourishing of the arts. Cultural traditions merged, as an introductory section of this exhibition shows. A series of images demonstrates the way in which the simple, strong colours flatter patterns and bold designs of a lively local idiom meld with the more sophisticated palette, the interest in portraiture and multiple perspective points of a fundamentally Islamic aesthetic to create an innovative new style. This flourished under the auspices of rulers who understood the ways in which pictures could propagate their political and religious messages.

Don’t be deterred if this all sounds a bit specialist. You don’t need to delve into the details of stylistic development to enjoy this show. You don’t need to mug up in advance on the Ragmalas (musical compositions that gave rise to a series of paintings) or the Ramayana (an ancient Sanskrit epic), or leaf through a “who’s who” of Hindu deities. Of course, many of the images in this show — being illustrations of traditional stories — are complex, involving multiple scenes and milling casts of characters; and you may have to resort to the wall texts (there is also a sumptuously illustrated catalogue with useful close-ups of the most exquisitely detailed bits) to help you to get into the story. But, for the most part, you don’t have to read anything to be able to respond. These paintings work on a simple visual level. Here is an oasis for the imagination, a show to sweep you away on an intoxicating cosmological voyage.

The journey begins in the lush pleasure gardens created by Maharajah Bakhat Singh, who ruled from 1725 to 1751 from the splendid Persian style palace that he created, complete with complex hydraulic irrigation system, at Nagaur. The maharajah was clearly not a man of restrained tastes. Here is a plump, pale-skinned potentate enjoying a life of fleshly ease with his zenana girls: a harem of frolicsome, fan-waving, sloe-eyed lovelies (though early attempts at Moghul techniques of modeling mean that many of them look as if they are sporting a five o’clock shadow). They are seemingly prepared at any point to do anything, from massaging his feet through entertaining him with music to accompanying him to the waiting beds.

A life of unalloyed pleasure is captured in all its delectable detail: you can compare the palace architecture with a modern-day photograph, you can feel the milling panic of turbaned minions trying to stop stampeding elephants through the celebrations of Holi (a Hindu festival) in which clouds of powdery colour are flung boisterously about. In a wonderful bathing scene the pampered ruler playfully squirts his cavorting harem with a syringe. It doesn’t take a phone call to Vienna to work that one out.

During the reign of his successor, Maharajah Vijai Singh (1752-93), these intimate depictions of royal pastimes were transformed into transcendent visions. Lush gardens become the playground of Hindu gods in alluring illustrations that, painted on an unprecedently large scale, would have probably been held up by two imperial flunkies while someone else recited the stories that they told. As the stories of the Ramayana unscroll across surfaces, the palette blossoming into myriad vibrant hues, into trippy magentas and entrancing emeralds and psychotic tangerines, the spectator starts to feel as though he is on some hallucinogenic trip, entering some alternative world.

Traditionally Indian art imposes a vision of order. The conventional miniature translates the vast and chaotic realities of a vast tribal nation into tiny patterned squares. But these manuscripts seem not only quite literally to open up larger spaces, but to speak more explicitly of some more encompassing force beyond.

The show is broadly divided into three parts and as spectators reach its last section they leave earthly pleasures behind and enter the mystical realm of Man Singh. This maharajah, who ruled from 1803-43, became a devotee of the Naths, a yogic sect much given, the pictures suggest, to sitting around with their legs crossed and contemplating chakra points.

Hundreds of propaganda images were produced showing Man Singh, the supreme terrestrial ruler, offering humble obeisance to the immortal ascetic Jalandhar Nath. It is not that the opulent aesthetic to the court is lost. Painters could create works as delicate, as evocative and sometimes as erotic as their predecessors. But when it came to religion, whereas artists had focused on depicting incarnate deities, on gods who could take physical, if often rather unusual form, now painters were now called upon to speak of the profound mysteries that lay at the core of Nath teachings. They were called upon to depict the Absolute.

Perhaps the most extraordinary pieces — certainly in the sense of being the least expected — are those that look almost like modern-day abstracts. Look at the manuscript Three Aspects of the Absolute, for example. It is a series of three framed squares. The first is nothing but a field of shimmering gold, a piece of pure abstraction that evokes the eternal, formless unity of the original manifestation of the cosmos. The second shows a haloed holy man floating through these luminous expanses. In the third, he exudes a silvery light that divides the image into two, as cosmic matter is divided from consciousness.

To Nath devotees, seeking to discipline their bodies through yoga so that they could glide back and forth between the phenomenal and the supramundane this image would have made perfect sense. To visitors who have followed this show’s hallucinatory journey it can likewise make sense. Here is Nirvana hanging on a wall.

Gardens and Cosmos: The Royal Paintings of Jodhpur is at the British Museum, WC1 (020-7323 8299; www.britishmuseum.org), from tomorrow to Aug 23

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