Watch Lupin III: The Castle of Cagliostro Online

It is a cross over between the series Lupin III and Detective Conan and takes place after the television special Lupin the 3rd vs Detective Conan. The plot follows Conan Edogawa who sets out to apprehend Arsène Lupin III, the suspect of stealing a jewel called Cherry Sapphire.(Souce: Wikipedia)

In a country called Vesparand, a new mineral with super stealth properties has been discovered which attracts Lupin's attention. At the same time the untimely deaths of Queen Sakura and her son, Prince Gill, leave Princess Mira next in succession to the throne. During a trip to Japan, Princess Mira finds her double in Ran and takes the opportunity to trade places, which ends with Ran being spirited away to Vesparand with Conan, Zenigata, Kogoro Mouri, Lupin and Fujiko in tow. There they meet a waiting Jigen and solve the mystery of the deaths of the Queen and Prince. (Source: ANN)

Jigen tells the story of the gang's first meeting. Jigen, still a member of the Mafia, is hired to protect an ancient artifact - one that Lupin and Fujiko are both trying to steal. Meanwhile, Inspector Zenigata, then of the Japanese police force, has come to the U.S. on Fujiko's trail, and Ishikawa Goemon is searching for the lost treasure of his clan. (Source: ANN)

The special's story centers on the kidnapping of Rebecca Rossellini, a beautiful business owner who is a featured character in the new Lupin III TV anime series that is currently airing. Upon hearing of her kidnapping, Lupin rushes to her rescue. Additionally, Lupin receives a written challenge from a mysterious person named "The Masked Count" suggesting a contest: who can obtain the "inheritance of Count Cagliostro" first.(Source: ANN)