Sekai Saikou no Ansatsusha, Isekai Kizoku ni Tensei suru

Pretty young girls have been captured and are being sold off at an illegal auction. Marquess Collide is engaging in human trafficking, and all her buyers are evil nobles as well. By royal order, Lugh the assassin, Dia the mage, his assistant Tarte, and Maha the intelligence expert begin their investigation of Marquess Collide. However, Dia is captured by Marquess Collide and put up for auction--

The world's finest Assassin accepts the Goddess' request to assassinate the Hero. After defeating the Demon Lord and saving the world, the Hero will then use his power to destroy the world. The Goddess wants the Assassin to be reincarnated in another world and assassinate the Hero in order to save it. After being betrayed and killed by his client in his previous life, the Assassin accepts her offer in order to relive his life in a new world, but--

Lugh's magic teacher is a pretty girl named Dia. Being a few years older than Lugh, Dia treats him like a younger brother and sneaks into his bed every night in order to sleep next to him. Because he needs her to teach him magic, Lugh can't complain, but gradually he and Dia grow closer--

The goddess is formulating a plan to stop the Hero's rampage. After defeating the Demon Lord and saving the world, next he will use his power to destroy the world. The goddess has decided to summon the souls of professionals from other worlds and reincarnate them. She summons the soul of the world's finest special forces soldier who excels in combat and asks him to defeat the Hero, but--

Officially a family of renowned doctors and assassins behind the scenes, the house of Tuatha Dé is the right hand of the kingdom. Trusted to become his father's heir, Lugh assists his father with his mission. However, there are those who are displeased by Lugh's success. Lugh defends his status in a challenge against them--

A group of girls were toughing it out despite their impoverished circumstances. Though they had nowhere to live, they ran a business and enjoyed their lives together. However, an orphanage captures the girls and forces them to move into the facility. Motivated by subsidies, the orphanage puts the girls to work and even has them work as ladies of the night--

Lugh heads to the neighboring kingdom of Swoigel. Once a month, he travels to the Viekone region in order to visit Dia. During his visit, together Lugh and Dia develop new spells. Meanwhile, left back home by themselves, Tarte and Maha hold a tea party together. While Lugh is gone, they have a fun time preparing a special meal together, but--

With the success of his cosmetics brand Orna, by acquiring financial power and the Balor Trading Company's information network, Lugh has gained even greater strength as an assassin. However, since being reincarnated in this world, Lugh still has not completed a proper assassination. Then, Lugh's father Cian tells him that he doesn't have to become an assassin--

Lugh receives a request for assassination. He takes Talt with him and goes to a commercial city, Pisaal. He decides to gather information beforehand to justify whether the target needs to be assassinated. Once there, Lugh saw the suffering of the people that was caused by his target. It's a tragic sight as a young child is forced to sell drugs while her mother was emaciated and was also addicted to it. Thus, Lugh formulates an assassination plan based on the information that Maha had collected. He was successful in killing for the first time in this life, though he soon felt the weight of his action and became emotional after witnessing the wife of the target grieving.

In front of Lugh and Talt who've just finished a magical training camp in a deserted island, Maha appeared with an information. It's about how the Swoigel kingdom is being engulfed in a civil war. Lugh who was worried, head towards Dia, but Viecone territory is peaceful as always. Dia ignored Lugh worries, and suggested to go for a date.

Lugh got a assasination request for Dia. And the principal is Count Viecone, who is Dia's father and Cian's old friend. Cian expects Lugh can finish the assasination. But if he refuses it, Cian will do it with his own hands.

Lugh and Dia's father come up with a plan to fake Dia's death and safely sneak her into to the Tuatha Dé region. Unfortunately, the arrival of Setanta MacNess thwarts the plan before they can enact it. As Setanta is the wielder of Gae Bolg, Lugh knows that he will not survive a direct confrontation with him, so he fires a magnetically propelled tungsten projectile into the atmosphere and stalls Setanta long enough for it to obliterate him. Afterwards, the Tuatha Dé family adopts Dia under the guise of being Lugh's younger sister, completing the mission. Sometime later, Lugh receives word that the Hero, Lord Epona Rhiannon, has arrived in the Alvan Kingdom.