Sony currently holds the copyright to part of the Beatles catalog, including such hits as "Love Me Do" and "I Want to Hold Your Hand"—but Paul McCartney is suing to get the songs back. Per TMZ, the songs in question are the ones Michael Jackson once held the rights to; his estate sold the rights to Sony last year for $750 million. McCartney has been trying to get Sony to transfer the rights to him by 2018; as ABC News explains, he's been notifying Sony and the prior rights holders of his intention to reclaim the rights since 2008. A McCartney rep explains, per Rolling Stone, that the suit is attempting "to confirm his ownership" of the songs "which are granted to him by US copyright law."
As Rolling Stone and Variety explain, the US Copyright Act says that for works in which creators transferred their interests before 1978, rights can be reclaimed by the creators 56 years after the original copyright date as long as termination notices are served to the current rights holder; next year will mark 56 years since McCartney started writing songs with John Lennon, and his suit notes that at the time the songs in question were created, he and Lennon typically transferred their rights to publishers in exchange for royalties. The Hollywood Reporter, which says the case "could become one of the most important legal battles in the music industry this decade," has more detail on the ins and outs of the law.