"In Search of Lost Time" is the fifteenth episode of the third season of Supergirl, and the fifty-seventh episode overall. It aired on April 23, 2018.
M'yrnn inadvertently causes psychic disturbances at the DEO; Supergirl must work with J'onn to contain the chaos; Mon-El begins training Kara in fighting techniques he's learned in the future.
J'onn having had an imaginary friend from a parallel dimension named "Zook", a reference to the DC comic book character that appeared in comic books published by National Periodical Publications (now known as DC Comics) in the 1960s. He was the partner of the superhero J'onn J'onzz, the Manhunter from Mars (now known as Martian Manhunter).