Marvel Studios has decided on a title for the in-development Avengers 3, making official that the film will be called Avengers: Infinity War. Avengers 4, meanwhile, has been given the placeholder name Untitled Avengers.

Infinity War is the first installment of a two-part story that concludes with Untitled Avengers; the two films were announced together at an event in October 2014 as Avengers: Infinity War Part 1 and Avengers: Infinity War Part 2. Their respective release dates of 5/4/18 and 5/3/19 have not been changed.

Both films are being helmed by Anthony & Joe Russo, who directed Captain America: The Winter Soldier and Captain America: Civil War and were handed the reins of Marvel’s flagship franchise following the departure of Joss Whedon, who directed 2012’s The Avengers and last year’s Avengers: Age of Ultron, both of which are among the seven top-grossing films of all time worldwide. The Russos confirmed in May that Avengers 3 and 4 were being retitled in part to put to rest speculation that the two films were merely one large film split in half.

Widely believed to be inspired by the 1991 Marvel Comics miniseries Infinity Gauntlet, Avengers: Infinity War and Untitled Avengers has been described by Joe Russo as an event in which the entire Marvel Cinematic Universe up to that point will unite “to battle the greatest threat to the world and universe that you’ve ever seen.” The miniseries saw the Marvel Comics universe faced with doom when archvillain Thanos the Mad Titan, in love with/obsessed with the (female) physical incarnation of Death, assembles all six “Infinity Gems,” granting him godlike power over the universe. (For those keeping score, the gems control Time, Space, Mind, Soul, Reality and Power.) Believing she’ll reciprocate his feelings if he delivers enough souls to her, his plan is to kill half of all living things.

Thanos, played by Josh Brolin, has been established as a major behind-the-scenes player in the events of the Marvel Cinematic Universe. First seen in a voiceless cameo in the stinger to Avengers, where the obsession with Death was teased, he showed up again as a major supporting character in Guardians of the Galaxy and later in the stinger for Age of Ultron. As with the comics, the Infinity Gems — called stones in the MCU — have played a major role, factoring into the plots of Captain America: The First Avenger, The Avengers, Thor: The Dark World, Guardians of the Galaxy and Age of Ultron.

While the plot and casting have been kept largely under wraps, Robert Downey Jr (Iron Man), Chris Pratt (Star-Lord), Chris Evans (Captain America), Chris Hemsworth (Thor) and Elizabeth Olsen (Scarlet Witch) have been confirmed to appear in the films, with the assumption being that Mark Ruffalo, Scarlett Johannson, Samuel L. Jackson, and other longstanding Marvel cast members also will appear. The Russos also have teased appearances by Benedict Cumberbatch’s Doctor Strange as well as Brie Larson’s Captain Marvel, though Marvel Studios has clarified that Captain Marvel will debut first in a stand-alone film to be released between Infinity War and Untitled Avengers.