Centered around the worship of the Cromulons, Headism began soon after the first Cromulon appeared above Earth and caused drastic weather changes. Following the initial appearance, many people gather at the local church, including Jerry, Beth, and Summer, as well as Principal Gene Vagina. At this church, Principal Vagina declares, "The old gods are dead, fuck all previously existing religions. All hail the one true god: the giant head in the sky." He then exits the church to pray in the street.
While Principal Vagina prays, Rick and Morty perform "Get Schwifty" for the Cromulon, as he is actually casting for an intergalactic music contest called Planet Music. The Cromulon enjoys the song and leaves, causing those at the church to believe that Principal Vagina's praying pleased the Cromulon. Beth begins voicing doubts, but is cut off when Earth is teleported to another part of the universe to compete on Planet Music. With Cromulons now filling the sky, the rest of the churchgoers begin praying as well.
Soon after Headism begins to officially form, with Gene Vagina as its leader. The town appears to turn into a Headistic theocracy under Principal Vagina. They begin sacrificing those who they deem to be immoral by tying them to a bunch of balloons and letting them float up towards the heads. The first sacrifices are at the Ascension Festival, where they sacrifice three "unwantables", wearing signs which read "Thief", "Goth", and "Movie Talker".
Summer becomes obsessive about the religion, she is joined by her boyfriend Ethan who plays guitar and the two of them learn the 'seven contemplations of the head' by singing them.
Although Beth and Jerry seem opposed to this new religion, they enjoy the positive effects it has on Summer, who begins showing her parents much more respect, as well as performing chores such as cooking every meal. They begin feeling torn between liking this new respect and missing their daughter's old personality.
Beth and Jerry eventually decide on attempting to remove Summer from the religion, but are labeled blasphemers and sentenced to be sacrificed. Elsewhere, Rick attempts to perform a new song but fails. This angers the Cromulons, which the Headists interpret as disapproval of Beth and Jerry's sacrifice. Summer and Ethan then save Beth and Jerry. Further misunderstanding lead to Principal Vagina being removed from his position as church head. Meanwhile, Morty joins Rick to successfully perform a song and impress the Cromulons. Earth is declared the final winner of Planet Music. After hearing this announcement, the Headists realize that they had been misinterpreting things the whole time, resulting in the dissolution of the Headist church.