D4DJ Double Mix is a self-contained special episode centering around the groups Merm4id and RONDO. 

"D4 FES.", a DJ festival that has been described as legendary, was relaunched, and Merm4id's long-cherished wish to participate came true. Marika Mizushima, Saori Hidaka, and Dalia Matsuyama are concerned about the unusual behavior of Rika Seto during the swimsuit photo shoot. Meanwhile, RONDO's Tsubaki Aoyagi is alone in the chapel, indulging her hopes for the future how their world will be expanded.
 
One day, after a live concert at the clubhouse ALTER-EGO, Rika rushes into Tsubaki's place where she is with Nagisa Tsukimiyama, Hiiro Yano, and Aoi Miyake. She, who overcomes everything with luck and charm in order to enjoy life, makes a surprising proposal. 
(Source: Crunchyroll, edited)