After Jonathan's death, his son, George Joestar II (Jorge Joestar) travels with Erina to the Canary Islands in Spain on the island of La Palma. There, he pledges his love to Elizabeth Joestar (later Lisa Lisa) and eventually grows up to become a World War I Air Pilot.
In another story, kid detective Joji Joestar sets out on a bizarre quest to solve a murder in Morioh, which slowly turns into a universe-hopping adventure after the island begins moving on its own.
The information below derives from a source not written by Araki. As such, it may or may not be considered canon.
The odd-numbered chapters of the story tell of how Jorge, Elizabeth, and another girl named Penelope de la Rosa grew up together, eventually leading to Jorge joining the Royal Air Force and marrying Elizabeth. However, this time around, their wedding is threatened by a zombie invasion. Along the way he makes new friends, such as Tsukumojuku Kato and Steven Motorize, and gets framed for a murder.
The other chapters, however, are another story altogether and considered the true story of the novel. Set in the 20th century after the world has been reset (similar to the effects of Made In Heaven) exactly thirty-six times, it focuses on kid detective Joji Joestar, who while investigating a recent string of incidents comes across a boy detective named Tsukumojuku Kato, who has somehow traveled between universes. After he is found dead in Morioh, seemingly by Yoshikage Kira's hands, Joji heads to investigate. Along the way, Joji eventually reaches outer space, and meets 36 versions of Kars, each from a different universe, one of which accompanies him back to Earth, all ending in a massive fistfight with Dio Brando as he attempts to become the Holy Corpse and gain unlimited power.
New Stands
In addition to the Stands from the original manga such as King Crimson and Dirty Deeds Done Dirt Cheap, JORGE JOESTAR introduces several new Stands that are all named after foreign (to Japan) films. There are also three new Stand-like abilities that are not truly Stands but have the same power:
Wounds (ウゥンド, Wundo) are created from repeated trauma to the afflicted's mind and body; they are less controllable than Stands are, and tend to serve a purpose of stopping the user's source of pain.
Bounds (バウンド, Baundo) are forces taken directly from the environment that the user manipulates.
Beyonds (ビヨンド, Biyondo) are, as their name implies, abilities that extend beyond time and space.
The 36 Kars living on Mars are presumably the 36 souls of sinners that DIO required for his plan to reach Heaven.
The DIO of Part 3 was not the real Dio Brando but rather Giorno Giovanna with a part of Dio's soul inside his body after the real Dio Brando becomes the Ultimate Life Form.
The Saint's Corpse of Steel Ball Run are actually parts of Dio's body, which Jonathan Joestar soon combines with after being resurrected.
Morioh and Passione's island headquarters both suddenly sprout legs from their foundations. They split from the mainland and sail across the sea. The islands eventually encounter one another, which is how the heroes are able to meet one another. The same eventually happens to England and Manhattan Island.
Trivia
As Erina and George II move to the Canary Islands, he is renamed "Jorge", the Spanish equivalent of "George", despite its completely different pronunciation in Spanish (xoɾxe; "Hor-hay").
As far as JoJo's Bizarre Adventure as a series goes, the JORGE JOESTAR novel is considered the most disconnected from the events of the canon, with its own exclusive plot elements and characteristics.
The novel seems to rely heavily on a meta-narrative, often making very plot significant allusions to the fact that it is a work of fiction, and frequently hinting at Maijo's other works that contain the same themes.
Despite no official English translation of the novel being available, Jorge Joestar recieved an English fan-translation by BuddyWaters in 2017
"Thank you, thank you! I'm Denta Shishimaru, the mayor of Morioh!" (P.234)
There is a door. A door in the floor. (P.238)
NSFWTAG
Translation notes
The English fan translation of JORGE JOESTAR refers to U-Boat by the name "Das Boot", after the film. The original Japanese text does not feature the Latin text "Das Boot", unlike the other new Stands named after films whose titles differ greatly between the Japanese and English titles.