It sounds like something out of a movie - Russian oligarch accuses Swiss art dealer of swindling him out of US$1 billion (S$1.34 billion) on world-class art.

A six-year legal battle plays out in the courts of Singapore, Hong Kong, New York, Monaco and Geneva. Throw in some eye-wateringly expensive masterpieces, including a mysterious painting thought to be a lost work by Leonardo da Vinci, and you have "The Bouvier Affair".

The entanglement between Swiss businessman Yves Bouvier, 58, and Russian fertiliser magnate Dmitry Rybolovlev, 54, has been described as one of the biggest legal battles the art world has seen.

Mr Bouvier, who settled in Singapore in 2009, told The Sunday Times it even prompted him to try to sell Le Freeport, a high-security vault in a duty-free zone near Changi Airport.

But the battle has now turned in his favour. In September, the Geneva public prosecutor's office dropped the last outstanding criminal case initiated by Mr Rybolovlev against Mr Bouvier, which included charges such as fraud and money laundering.

This was a "complete victory" and "the end of a six-year nightmare", Mr Bouvier said in a statement.

The epic legal battle has cost him an estimated US$50 million in legal fees since 2015, he tells The Sunday Times in a Zoom call from Geneva.

Mr Bouvier had been trying since 2017 to sell Singapore's Le Freeport - which he co-owns - for cash, and because of the impact of the spat on his business.

Previously known as Singapore FreePort, it opened in a duty-free zone in 2010 with support from the Singapore Economic Development Board (EDB). It is used by the wealthy to store art and other prized possessions.

On his plans for the business now, Mr Bouvier says: "I am considering all options in the best interests of the free port."

The fight between the two men is not over yet. Mr Rybolovlev has appealed the ruling, while Mr Bouvier has started preparing countersuit proceedings.

"I had to stop almost all art dealings, logistics and transport activities to defend myself against the massive attacks during the last six years," says Mr Bouvier, who spends his time in Singapore, Paris and Geneva.

"I value the loss and damages suffered to be several billion Singapore dollars and have commenced legal action in Singapore to recover my losses."

The Swiss dealer, who speaks imperfect English and occasionally gets help from an interlocutor during the interview, is known for having exported the Geneva concept of a free port - a tax-free storage facility for art and other valuables - to Singapore and Luxembourg. He co-owns Le Freeport in both countries.

The saga began in 2002, when Mr Bouvier and Mr Rybolovlev first met at Geneva Freeport.

Mr Rybolovlev, a cardiologist who made his fortune in fertilisers, was supposedly unhappy at the time because a Marc Chagall painting he bought did not have an authenticity certificate, The New Yorker reported in 2016.

Mr Bouvier, whose shipping and storage company Natural Le Coultre was located at the free port, introduced himself, gave the previous owner a call and helped Mr Rybolovlev get the certificate.

This was the start of a relationship between the two men. For more than 10 years, Mr Bouvier - who communicated with the Russian via a translator - helped the billionaire build up his collection. This included 38 pieces of art that cost US$2 billion.

In 2015, Mr Rybolovlev accused Mr Bouvier of having inflated the prices of the works sold to him. He claimed that Mr Bouvier had swindled him out of US$1 billion on works which included masterpieces by artists such as Amedeo Modigliani, Paul Gauguin and Vincent van Gogh.

One famous item - the subject of a documentary, The Lost Leonardo (2021), by Danish director Andreas Koefoed - was Salvator Mundi, a painting of Jesus Christ that has been widely attributed to da Vinci.

In 2013, Mr Bouvier bought it for US$80 million from dealers in New York in a sale brokered by auction house Sotheby's, which would later be dragged into the dispute. He hired a friend, a former poker player, to help negotiate a better price, saying: "He knew how to read the feelings of the seller better than me."

Mr Bouvier then sold it to Mr Rybolovlev for US$127.5 million - a mark-up of over 50 per cent.

In 2017, Mr Rybolovlev sold the painting for a record US$450 million at a Christie's auction. The buyer is believed to be a Saudi Arabian prince, and the painting's whereabouts are a mystery - in 2019, media reports suggested it might be on his yacht.

Mr Rybolovlev is known for his lavish property purchases - including a Greek island, as well as former United States president Donald Trump's mansion in Florida. In 2014, he made the news for a multibillion-dollar divorce settlement.

Mr Bouvier, The Straits Times previously reported, acquired No. 6 (Violet, Green And Red), a painting by abstract expressionist Mark Rothko, for US$80 million and sold it to Mr Rybolovlev - who is also president of AS Monaco football club - for €140 million (S$219.2 million) in 2014.

In February 2015, he made his way to Mr Rybolovlev's Monaco penthouse, assuming they were to discuss the outstanding payment. Little did he know, the Russian - via two companies he used to buy art acquired by Mr Bouvier - had filed a criminal complaint accusing him of fraud. Upon arrival, Mr Bouvier was arrested and detained for three days before being released on bail.

The dispute between Mr Bouvier and Mr Rybolovlev made its way to Singapore courts in 2015, when the Russian tycoon called for a worldwide freeze of Mr Bouvier's assets, as well as those of Ms Tania Rappo - a friend of the Rybolovlev family who took commissions from Mr Bouvier from the sale of art to Mr Rybolovlev.

The High Court granted the injunction and ordered the Swiss dealer to hand over the Rothko painting. The assets were later unfrozen by Singapore's Court of Appeal. In 2016, the High Court dismissed an attempt by Mr Bouvier to suspend the civil suit filed here. In 2017, the Court of Appeal overturned an earlier judgment and ruled - in Mr Bouvier's favour - that the dispute should be heard in Switzerland, not here.

During the legal fight in Singapore, a key contention was over the nature of the relationship between the two men - whether Mr Bouvier was an agent who owed fiduciary duties to two companies Mr Rybolovlev controlled, or an independent seller who was entitled to mark-ups.

Mr Bouvier was represented by senior counsel Edwin Tong. Mr Rybolovlev was represented by senior counsel Alvin Yeo and, later, senior counsel Davinder Singh.

Monaco's Court of Appeal in 2019 dismissed all the charges of fraud and money laundering.

But by then, the case had sparked "Monacogate", a large corruption scandal involving an investigation into whether Mr Rybolovlev had used perks to woo officials as allies in his feud. Earlier this year, the Swiss authorities also opened a criminal probe concerning the Russian tycoon's involvement in Mr Bouvier's arrest in Monaco.

Mr Bouvier says the legal proceedings these past six years have had a severe impact on his businesses.

"Before the case, I was in conversation with Beijing, Shanghai, Hong Kong, Macau... I had numerous projects on the table, ready to open, to realise in the world. But after (the case in 2015), I was no longer able to develop any of these projects."

In 2017, he sold his family business Natural Le Coultre - a company his father bought in 1982 - to French shipping firm Andre Chenue for an undisclosed sum. He also tried to sell Singapore's Le Freeport, which was later reported to have racked up $18.4 million in losses in about a decade up to 2018.

"In my free-port business in Singapore, I have lost 30 per cent of turnover since 2015. It was difficult to maintain my business," he says.

His attempts to sell Le Freeport failed. Last year, he sued a group of businessmen whom he alleged had agreed to buy it for $83.3 million, before repeatedly delaying and finally reneging on the deal. He has dropped the case, he adds.

Mr Bouvier says that No. 6 (Violet, Green And Red) - one of the most expensive paintings in the world - remains under seal at Le Freeport here.

He adds that in Singapore, he had financially supported projects such as Singapore Pinacotheque de Paris at Fort Canning, contemporary art fair Art Stage and Art Plural Gallery in Armenian Street.

These are no longer running. Singapore Pinacotheque de Paris, a private museum, closed in 2016 after barely a year in business. Several lawsuits were filed against Art Heritage Singapore, the company behind it.

Art Stage Singapore in 2019 was notoriously cancelled nine days before its planned opening, with the National Arts Council, EDB and Singapore Tourism Board saying then that they understood it to be a "commercial decision".

Mr Bouvier says he is working on two tell-all books on his life and legal battles, and that movie producers have approached him for the rights to make a film about his life.

"Perhaps I will sign, I don't know."