Sea of the Beginning is the eighth part of the Kaiten Mutenmaru series, serialized in the official site of the franchise from August 12, 2011, to February 10, 2012, for a total of 13 chapters. It was published in two books by Micro Magazine on June 27, 2014. It introduces the readers to the concept of the Abominable Crystal and the first main characters of the second season.
The peace of Bikkura Village is shattered when a foreigner (named Shinuka To'omotta in supplementary material) turns into a monster. With the help of the fiery lion Kura Shishi no Mikoto, Mutenmaru manages to save both the village and the foreigner. He decides to travel by ship when the foreigner reveals that he's come from the United States to find someone with the power to remove the corrupting influence of the Abominable Crystal.
The enemies in Mutenmaru's dream attack his ship and throw him down, leaving him to be found by mermaid Umimi Sheela on the Isle Whale three days later. Mutenmaru and Umimi encounter a group of poachers that target rare creatures, having already captured the Isle Whale. Mutenmaru uses up all of his energy to summon Kura Shishi and defeat the transformed leader of the poachers. Luckily, Umimi's Song of Healing awakens the water spirit Tear Crown, who releases the Isle Whale while driving off a giant shell monster and the rest of the poachers. Having dreamed of adventuring, Umimi joins Mutenmaru.
Cat Girl Siam Merluza kidnaps Mutenmaru's companions in the United States, promising to release them if he either gives her all his money or defeats her in a duel. The duel is interrupted when a monster throws one of Siam's men from a building, after which Mutenmaru and Siam defeat the monster together.
Mutenmaru's group arrive at Megapolis, where heroes collect Abominable Crystals from the monsters they fight. Aspiring hero Shinobu Kajika makes an entrance in an attempt to help Anchor 4 defeat a fiery blue monster in black. It's defeated instead by a disguised Crocell Xylel, who creates costumes for heroes as the CEO of Megadis.
Sister Meer and Father Brasse take Mutenmaru's group and Shinobu to a small church, where they care for orphans. Father Brasse reveals to Mutenmaru that both of the clergy people used to work in Megadis but escaped when they found something wrong with Crocell. The old man points out how all the monsters in Megapolis have appeared around Megadis, leading Mutenmaru's group and Shinobu to search the truth.
Crocell and scientist Thia Venta Zoa reveal that they are creating monsters so heroes can constantly provide hope for people. Transforming himself with an Abominable Crystal, Crocell challenges Mutenmaru to a duel. Sister Meer and Father Brasse deliver a surprise attack to Crocell in the duel, preventing Mutenmaru's potential death. Mutenmaru purifies Crocell of the Abominable Crystal with the radiant blade of his sword.
Thia blows up the building of Megadis, saying that the company has outlived its usefulness, but Anchor Sam leads many heroes to the aid of people. Crocell's friend and reporters reveal themselves as the enemies in Mutenmaru's dream, one of whom calls himself Sick. They disappear to the north, but not before Sick tells Mutenmaru that monsters will be around until his defeat. Father Brasse tells Shinobu to join Mutenmaru and become a true hero. The part ends with Thia discovering a baby boy born from a large Abominable Crystal in the remains of Megadis.
An extra chapter in Volume 2 called "Extra Story" depicts how Thia names the boy Kid.
Sea of the Beginning provides examples of:
- Dreaming of Things to Come: The part begins with Mutenmaru having a dream where he fights the forces of the Abominable Crystal alongside his future companions.
- Engineered Heroics: Crocell Xylel is seen in the television promising to wipe out monsters in Megapolis with costumes he creates for heroes as the CEO of Megadis. When Mutenmaru's group and Shinobu infiltrate Megadis to verify Father Brasse's suspicion, Crocell reveals that he's the one who creates monsters in Megapolis with the help of Thia to keep heroes fighting.
- Humiliation Conga: Crocell goes through a series of events that utterly demolishes his arrogance after Mutenmaru and his friends confront him. As the battle with Mutenmaru goes on, Crocell's ultimate hero suit transforms him into a grotesque monster who rants about how he will be the strongest hero in the world. Just as Crocell is defeated and lies defenseless on the ground, Thia betrays him by blowing up the building of Crocell's company to continue his own research. Crocell has to be saved by a group of heroes, one of whom helps him repent. Soon afterward, three more of Crocell's benefactors reveal that Crocell was just a pawn in Sick's plan instead of being friends and equals with him. This culminates in Sick flying away with Eryth and Ritol as he steals Crocell's research on Abominable Crystals for himself.
- Older Hero vs. Younger Villain: Father Brasse is a Miniature Senior Citizen who nonetheless wields his former powers as the legendary hero Master Megalo to stop Crocell Xylel, the young Corrupt Corporate Executive of Megadis.
