At the start of the Volume, Darkness receives a message and soon after leaves to visit her home. Several days later she returns and bugs Kazuma into doing a quest to slay a Legendary Monster. They initially fail the quest, but Darkness gets stubborn and she returns everyday to challenge the monster again and again. They eventually do defeat it, but to defeat the monster Kazuma enlists the help of the other adventurers at the Adventurer's Guild. When she sees the adventurers banding together to help her out Darkness relents and thanks Kazuma and the other adventurers for their help. She then disappears and leaves a letter to the party saying that she's thankful for the times they had and that she had to leave the party. It is revealed later that House Dustiness was in massive debt due to the Dustiness house having paid the money for repairs due to Aqua's flood at the start of series, and for the damages incurred with the Destroyer even though without Kazuma's party, everyone would have lost their homes.
For the sake of the people, Darkness took the debt from Axel's landlord Alderp (who was initially supposed to be the one to pay for all damages as he's the noble in charge of Axel) with a condition - if Darkness's father can't pay due to illness or some other reasons then Darkness will have to pay with her body. Due to the above condition, when Darkness's father fell ill, she had to marry him.
However, as Darkness did not tell anyone else of this arrangement, Kazuma, Megumin and Aqua were initially confused at first as to why she suddenly left the party. Due to these situations, Kazuma snuck into Darkness's house to find out some of the reasons why she left the party, and since she resigned her self to marry Alfred she asked Kazuma to 'climb the stairs of adulthood together', but due to Kazuma freezing up and commenting on her rock hard abs she flipped out and went into a murderous rage and ordered her servants to capture him so she could beat him up. In the ensuing chaos kazuma hid in Darkness's Fathers room where it was revealed that he was sick and he told Kazuma to run away with his daughter to make her happy. Darkness then barges in and attempts to capture Kazuma, but he escapes, albeit in an uncool manner with Darkness laughing at him, much to his infuriation. Kazuma sulks, but decided to listen to advice from Vanir to create products from his old world to sell and later he sold all his ideas to Vanir when he found out all the information needed (which was all according to Vanir's plan) and made a mess in her marriage, kidnapping her after paying back all the debt Darkness owed and claiming that Darkness was now his property since he paid for her and that he would have her 'pay her back with her body'.
After that they went to Dustiness mansion where Aqua removed the devil's curse that was on Darkness's Father. The reason for Darkness Father's illness is some curse from the Holy Relic that Eris was looking for in the mortal world, which was in Alderp's hands.