​Like many in her class, Koyuki Honda looks forward to her graduation ceremony. However, while her peers have family to attend, Koyuki has no one, as her only family—her brother—died years ago. While visiting his grave, she meets two brothers named Kiri and Victor Putin chasing a strange creature. They try to assure her that they are just actors shooting a movie; Victor, however, piques Koyuki's curiosity about the old legend of the clock tower at the cemetery: if you make a wish under the clock tower at midnight during a full moon, time will turn back, allowing someone to reclaim what they have lost. Deciding to make her wish, Koyuki learns the horrifying truth: the legend is a ruse for time demons called "Horologues" to steal humans' time. As they are drawn to regrets, Victor uses Koyuki as bait to lure a demon there. Kiri and Victor explain that they are Chronos Rulers, warriors who fight against the Horologues using the "Oath of Time"—an item allowing them to speed up or slow down time. With the demon destroyed and the crisis averted, Koyuki is able to go on her way free from regrets. However, she is not the only human with sorrows. The two boys must continue to hunt down Horologues in order to keep humanity safe. [Written by MAL Rewrite]

In this sequel, a girl named Kazusano Kazuki joins up with the 6 girls and Ikko. With Kazuki around, Ikko will find himself in more embarrassing situations with these priestesses-in-training. (Source: ANN)

Short specials included on the BD/DVD volumes of "Sekai de Ichiban Tsuyoku Naritai!"

The words that Gold Roger, the King of the Pirates, uttered just before his death excited the masses and the world has entered a Great Pirate Era! There is a group of young pirates who are about to set sail on the Grand Line. Monkey D. Luffy, a rubber man. Roronoa Zoro aka “Pirate Hunter.” Usopp, a sniper. Sanji, a seafaring cook. And “Cat Burglar” Nami. They, the Straw Hats, all place a foot upon a barrel and make their vows before their next journey across the great ocean. Luffy shouts “In order to be the King of the Pirates!” A decade ago in the Windmill Village... A little boy Luffy was enthralled with a pirate boss Red-Haired Shanks. But a group of mountain bandits shows up and makes fun of the pirates. “Why didn't you fight them?!” Luffy yells out angrily but Shanks says that it's nothing worth getting mad over. At that time, Luffy snatches the Gum-Gum Fruit from a treasure box and eats it and as a result, his entire body becomes rubber and he loses the ability to swim for the rest of his life! A few days later, Luffy is surrounded by the mountain bandits again and Shanks comes to help. “No matter what the reason, anyone who hurts my friends has to pay!!” Shanks and his pirate crew are incredibly strong and they beat down the mountain bandits. Higuma, the head of the mountain bandits, runs away kidnapping Luffy and heads for the sea. However, they encounter a local Sea Monster there and are attacked. Luffy is in a desperate situation... But again, Shanks saves the day. He outstares the Sea Monster and chases it away but in exchange, he loses his left arm... When Shanks leaves the village, he leaves his straw hat with Luffy. “Come bring it back to me someday! Once you've become a great pirate!” 10 years later, Luffy has grown up strong and he sets out for an adventure on a small boat. He again encounters the local Sea Monster for that fateful day. However, Luffy takes it down with a single blow of Gum-Gum Pistol. Luffy’s journey to become the King of the Pirates now begins! (Source: Crunchyroll)