My Childhood Friend Who I Used To Train Swordsmanship With Became A Slave, So I, As An S-Rank Adventurer Decided To Buy Her And Protect Her.

S-Rank adventurer and second-generation sword saint, Ryunoa Styler, is reunited with his childhood friend, Aine, who has been reduced to slavery. Ryunoa purchases Aine in order to save her, however, she's been given a cursed collar that causes the wearer to go into 'heat' once a day... Ryunoa's battle to find a way to remove the collar starts now!