Fear, survival, instinct. Thrown into a foreign land with nothing but hazy memories and the knowledge of their name, they can feel only these three emotions resonating deep within their souls. A group of strangers is given no other choice than to accept the only paying job in this game-like world—the role of a soldier in the Reserve Army—and eliminate anything that threatens the peace in their new world, Grimgar. When all of the stronger candidates join together, those left behind must create a party together to survive: Manato, a charismatic leader and priest; Haruhiro, a nervous thief; Yume, a cheerful hunter; Shihoru, a shy mage; Moguzo, a kind warrior; and Ranta, a rowdy dark knight. Despite its resemblance to one, this is no game—there are no redos or respawns; it is kill or be killed. It is now up to this ragtag group of unlikely fighters to survive together in a world where life and death are separated only by a fine line.

A hundred years after catastrophic shifts in tectonic plates caused most of its area to submerge, Japan now thrives as a maritime nation. High functioning cities line the coasts of Japan, and well developed sea routes are defended by specialized naval ships. The marine industry is flourishing, and young girls are eager to join the Blue Mermaids—highly skilled women who devote their lives to protecting the seas. After some time apart, childhood friends Akeno Misaki and Moeka China are reunited at Yokosuka Girls' Marine High School. Here they are joined by others who share their dream of becoming Blue Mermaids. Soon, training exercises commence, with Akeno and Moeka being put in charge of their own ships and crew. Everything is progressing smoothly for Akeno as she settles into her role as captain and builds camaraderie between her fellow crew members aboard the Harekaze, a destroyer ship. However, unexpected engine and navigation failures soon leave their ship stranded and the crew in danger. High School Fleet follows the Harekaze's valiant crew as they navigate treacherous waters and other obstacles on their quest to become fully fledged Blue Mermaids.

Born from a cocoon in the village of Old Home, a young Haibane—a being with a halo and small gray wings—awakens to a world she does not understand without memories of her past. Named Rakka for the dream of falling she had while inside the cocoon, she soon becomes accustomed to life in the strange town. However, there are strict rules for the Haibane, such as being forbidden to leave the village or go near the walls surrounding it. These, along with mysterious disappearances of their kind on their Day of Flight, begin to unsettle Rakka and the others since they know almost nothing about their own kind. As Rakka and the other Haibane live their lives with no memories of the past, they try to break free from their former pain and ultimately find salvation.

Following their participation at the Inter-High, the Karasuno High School volleyball team attempts to refocus their efforts, aiming to conquer the Spring tournament instead. When they receive an invitation from long-standing rival Nekoma High, Karasuno agrees to take part in a large training camp alongside many notable volleyball teams in Tokyo and even some national level players. By playing with some of the toughest teams in Japan, they hope not only to sharpen their skills, but also come up with new attacks that would strengthen them. Moreover, Hinata and Kageyama attempt to devise a more powerful weapon, one that could possibly break the sturdiest of blocks. Facing what may be their last chance at victory before the senior players graduate, the members of Karasuno's volleyball team must learn to settle their differences and train harder than ever if they hope to overcome formidable opponents old and new—including their archrival Aoba Jousai and its world-class setter Tooru Oikawa.