In a post-apocalyptic world, humanity is pushed to the brink of extinction by the constant threat of giant creatures known as klaxosaurs (叫竜, kyoryū), which are subdivided into at least four categories based on their size: “Conrad,”[d] “Mohorovičić,”[e] “Gutenberg,”[f] and “(Super) Lehmann.”[g] Parasites are raised to pilot giant mecha known as Franxx[h] (フランキス, Furankisu) in boy-girl pairs. A male parasite is termed a “stamen,” and a female parasite is referred to as a “pistil” (the male and female reproductive parts of a flower, respectively). Parasites are artificially created and have short lifespans.

A team of ten parasites is assigned to the experimental Squad 13 of Plantation 13. One of them, Hiro (Code:016), is a former pilot-candidate prodigy who can no longer synchronize with his partner, resulting in their failure to complete the training program. While skipping his squad’s graduation ceremony, Hiro encounters Zero Two (Code:002), an elite Franxx pilot with klaxosaur blood, red horns, and an infamous reputation as the “Partner Killer.” It is rumored that none of Zero Two’s partners have survived pairing with her more than three times. Shortly after, a Klaxosaur attacks, disrupting Hiro’s graduation ceremony and resulting in Zero Two’s partner being killed in action. Zero Two, left without a partner, allows Hiro to ride with her and Hiro becomes Zero Two’s “darling”. Later in the series, he becomes her partner.