From the Anime Cinematic Universe comes a new chapter featuring heroes with bizarre abilities: JoJo's Bizarre Adventure: Diamond is Unbreakable. In the aftermath of Tales of Zestiria the X: Civil War, Jotaro Kujo grapples with the consequences of his choices as both a superhero and a father. As he struggles to rebalance his home life with his responsibilities as a Stand User, he’s confronted by Holly Joestar and Joseph Joestar with an urgent new mission. Jotaro must once again summon his Stand, Star Platinum, and learn to fight alongside Holly and her Stand, The Passion, as the team works together to uncover secrets from their past.
In 1987, Suzie Q uses Mystic Artes to shrink between the molecules of a Rolance magical missile, disabling it but entering the microscopic Quantum Realm. Joseph Joestar raises their daughter Holly believing that Suzie Q is dead.
Years later, former criminal Jotaro Kujo takes up the mantle of a Stand User and discovers a way to both enter and return from the Quantum Realm using Time Stop. Joseph and Holly begin work on repeating this feat, believing they may find Suzie Q alive. Jotaro and Holly also start a romantic relationship and begin training to fight together, until Jotaro secretly helps Shepard Sorey during a skirmish between the Jump Force in violation of the Hellawes Accords. Jotaro is placed under house arrest, while Joseph and Holly go into hiding.
Two years later, Joseph and Holly briefly manage to open a tunnel to the Quantum Realm. Jotaro receives an apparent message from Suzie Q, who he is quantumly entangled with. Despite having only days left of house arrest, Jotaro decides to call Joseph. Holly kidnaps Jotaro, leaving a decoy so as not to arouse suspicion from the Amestrian agent Okuyasu. Seeing the message as confirmation that Suzie Q is alive, Joseph and Holly work to create a stable tunnel so Holly can take a vehicle into the Quantum Realm and retrieve Suzie Q.
Holly arranges to buy a part needed for the tunnel from the black market dealer Kira Yoshikage, however, Kira has realized the potential profit that can be earned from Joseph and Holly's research and double-crosses them. Holly fights Kira and his men off, until she is attacked by a ghost girl, referred to as the Reimi. Jotaro tries to help fight off Reimi, but she escapes with Joseph's portable lab.
Joseph reluctantly visits his estranged bastard son Josuke Higashikata who helps them locate the lab. Reimi restrains Jotaro, Holly, and Joseph when they arrive. Her father, a former partner of Joseph's, accidentally killed himself and his wife during a quantum experiment that caused Reimi's unstable state. Josuke reveals that he has been helping Reimi after the accident, who they plan to cure using Suzie Q's quantum energy. Believing that this will kill Suzie Q in the Quantum Realm, Joseph refuses to help them and the trio manage to escape.
Opening a stable version of the tunnel this time, Joseph and Holly are able to contact Suzie Q who gives them a precise location to find her, but warns that they only have two hours before the unstable nature of the realm separates them for centuries. Kira learns their location from Jotaro's business partners Kakyoin, Avdol and Polnareff, and informs a contact in the Amestrian government. Kakyoin warns Jotaro, who rushes home before Okuyasu can see him breaking his house arrest. This leaves Joseph and Holly to be arrested, and for their lab to be taken by Kira.
Jotaro is soon able to help Joseph and Holly escape custody, and they steal the lab back from Kira. Jotaro and Holly distract Reimi while Joseph enters the quantum realm to retrieve Suzie Q, but end up fighting Kira and his men allowing Reimi to begin the process of taking Suzie Q's energy. A car chase ensues across the streets of Amestria, ending at a pier. Jotaro takes the lab and enlarges it, while Kira and his goons are apprehended.
During this, Joseph encounters Suzie Q in the Quantum Realm, and arrive safely from the Quantum Tunnel. Reimi attempts to sabotage their return, but when Jotaro and Holly intervene, she is stopped. Suzie Q voluntarily uses some of her Hamon empowered by Magical Artes on Reimi to temporarily heal her. Jotaro returns home once again, in time for a suspicious Okuyasu to release him at the end of his house arrest. Reimi and Josuke go into hiding, but Joseph, Jotaro, Holly, and Suzie Q plan to harvest quantum energy to continue helping Reimi.
In a mid-credits scene, Joseph, Holly and Suzie Q all disintegrate in tandem with the Battle of Shura, while Jotaro is harvesting more quantum energy; without anyone to let him out, Jotaro remains stuck in the Quantum Realm trying to use Time Stop to escape. In a post-credits scene, the television in Jotaro's house is broadcasting a state of emergency when Amestria appears to be deserted.