FL grew up treated very coldly by her father, the King, who had wanted a male heir instead. The knight Eric was the only person who was ever kind to her, so she always loved him. As an adult, FL was about to be married off in a political marriage when her father suddenly died. So FL unexpectedly inherited the throne and now had more freedom in choosing whom to marry. She chose Eric.

Their marriage is happy and drama-free, except for the fact that there is an old royal tradition that the King's bride must be tied down for the whole first week of marital relations. This tradition had been created because there was once a bride who'd gotten too nervous and accidentally kicked a King in the face on their wedding night. So a tradition of always tying down the bride was established. However, this tradition had never anticipated there might someday be a female ruler. But FL successfully follows the tradition by tying down her husband instead for a week.

Just when life seems to be going well, a stranger shows up claiming to be the illegitimate son of the former King, which would make him FL's half-brother. As proof, he displays a special birthmark which only the royal line inherits. Problem is, FL can't find any birthmark like that on herself. So FL's claim to the throne is now in jeopardy until it's discovered that the supposed brother's royal birthmark was actually a tattoo and not real, so he wasn't really the King's son.

Also, it turns out that FL actually did possess the royal birthmark, only it happened to be concealed in a place that is rarely visible (that is, her buttcrack). So FL couldn't find her birthmark until she happened to engage in some snusnu activities in a revealing position. So, everything ended happily.

Thank you Aneliese for providing the spoiler