Kazuma's party went to Wiz's shop to say hello and discovered Yunyun hiding behind one of the shelves. It turned out that Yunyun hangs out at Wiz's Shop quite frequently, in hope that she may run into Megumin "by chance." After Yunyun got caught, she tries to pretend that she is a regular shopper and randomly grabbed the wish-granting choker from the shelves. As Megumin and Yunyun argued, the choker fell from Yunyun's hand and landed near Kazuma, who tries to ignore the minor chaos broke out around him and picked up the choker.
Reading the description "This choker will grant any wish.", Kazuma falsely believe it as a good-luck charm, and tries it on. Seeing this, Wiz soon told Kazuma about how the choker is a popular weight-loss tool among women and it forces the wearers to fulfill their own goals/wishes. The choker cannot be taken off by force and will gradually tighten to the point that it may kill the wearer in 4 days unless the he/she fulfill the stated goal in time.
Kazuma cannot remember what he wished for, so the ladies decided to help Kazuma by fulfilling any wishes he can think of, in order to save his life. They quickly learned that Kazuma's wishes are obnoxious and are all borderline sexual embarrassments, but they have no choice except to go along with his demands. The ladies went along with Kazuma's every wish despite their misgivings, but none of those is Kazuma's actual wish and the choker gradually tightens.
On the last day, Kazuma resigned to his fate and made peace. After the group hugged each other, they heard an audible click sound. Kazuma then realizes that his true wish was to have some peace and quiet, and that was fulfilled at last moment. No one noticed that the choker had been unlocked though, and Kazuma preceded to deliver his unflattering "apologies" to each of the ladies. All of those apologies are basically the honest but deeply offensive opinions Kazuma had. As Aqua shook Kazuma for his comments about her, they finally discovered the choker had already been unlocked.
As Aqua picked up the choker and "invited" Kazuma to try on the choker again, he reflected on his own words and his actions over the past days. The OVA ended with Kazuma sitting in Eris' reincarnation room, implying that he was either killed by the ladies directly or didn't survive his second try with the "wish-granting" choker.