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Credit for awesome summary Attack on Titan Season 4 Wikipedia.
Using the Paths dimension, Zeke shows Eren his life from birth through their father, Grisha's, memories, hoping to convince Eren that Grisha had brainwashed him. However, they discover that Grisha had, instead, forsaken his Eldian nationalism and allowed Eren to find his own path in life. Undaunted, Zeke states that with the power of the Founding Titan, he can still carry out the euthanasia plan regardless.
Zeke and Eren eventually arrive at the moment Grisha confronts the Reiss family. As Grisha pleads with Frieda Reiss to save Paradis with the Founding Titan, he reveals that the Attack Titan's ability is that the host can see the memories of its future inheritors.
When Frieda refuses to help, Grisha decides to kill the Reiss family and take the Founding Titan for himself but realizes that he cannot bring himself to do the deed. However, Eren goads Grisha into eventually slaughtering the Reiss family.
After the event, Grisha breaks down and sees Zeke, telling him that it will be Eren's plan, not his, that will come to pass. Grisha then expresses his regret to Zeke for not being a better father to him. He then pleads with Zeke to stop Eren who swiftly brings Zeke and himself back to the Paths dimension.
NOTE: This episode takes place in volume 30 of the manga.