Steins;Gate 0 Season 1 Episode 12

Mother Goose of Mutual Recursion: Recursive Mother Goose

Kagari collapses again after hearing Mayuri singing a song, which Rintaro believes may be a hint to regaining her memories. He finds that Mayuri had learned the song from Suzuha, who herself had learned it from Yuki in the future. Yuki then reveals she had learned the song from Rintaro's mother, who claims he was the one who sang it to her back when he first started his mad scientist phase. While visiting Mayuri's grandmother's grave for any clues, Kagari goes into a daze upon hearing a song on the radio and almost gets hit by a truck, after which she regains her memories of Mayuri being her mother. It is then revealed that Rintaro had learned the song from Kagari herself years ago, when a mysterious organization spotted her.