From the Super Mario Wiki
Queen Rotunda is a gluttonous, extremely large and somewhat self-delusional woman who is the apparent ruler of the kingdom of Rotunda Land. Queen Rotunda is a parody of the queen from Alice in Wonderland.
In The Super Mario Bros. Super Show! episode "Love 'Em and Leave 'Em", Queen Rotunda, desperate to gain the affection of an old friend, Prince Pompadour, had a hag named Hildy brew her a Passion Potion; Queen Rotunda's plan was to make Prince Pompadour drink this Passion Potion and make herself the first thing Prince Pompadour saw after drinking it, this would make Prince Pompadour fall madly in love with her.
It was shortly after Hildy completed her Passion Potion that Queen Rotunda was informed by one of her Beezo minions that thieves had been caught stealing her royal hot peppers. Outraged by the mere thought of someone stealing food from her, Queen Rotunda traveled to her throne room to see these thieves, taking some of Hildy's Passion Potion with her.
Arriving at her throne room, Queen Rotunda confronted her prisoners, Mario, Luigi, Toad and Princess Toadstool. Before deciding what should be done with Mario, Luigi, Toad and Princess Toadstool, Queen Rotunda decided to sample some of the cake she had her chef prepare for her impending wedding with Prince Pompadour, only to call it disgusting. After kicking the cart containing her cake away, Queen Rotunda sentenced Mario, Luigi, Toad and Princess Toadstool to be beheaded for stealing from her.
Before Queen Rotunda's sentencing could be carried out though, Mario managed to convince Queen Rotunda to allow himself to make her some "hot pepper pistachio ice cream" as the desert for her wedding; intrigued by the thought of tasting a new food, Queen Rotunda decided to allow Mario, Luigi, Toad and Princess Toadstool to live, at least, until after the hot pepper pistachio ice cream was made.
Later, after Mario had created the hot pepper pistachio ice cream, Queen Rotunda tried it, only to quickly be overcome by the sheer hot flavor of it; desperate for something to stop the burning in her mouth caused by the hot pepper pistachio ice cream, Queen Rotunda began to frantically hop around her throne room, asking for water. Mario, wanting to help Queen Rotunda, grabbed the nearby bottle of Passion Potion on the queen's throne and poured it into her mouth.
Unfortunately, Mario was the first thing Queen Rotunda saw after ingesting the Passion Potion, meaning she had fallen madly in love with him. After a short chase after a confused Mario, Queen Rotunda captured Mario by wrapping him in carpet and dragging him away to plan his and her wedding, after telling her Beezo guards to take Luigi, Toad and Princess Toadstool to her castle dungeons.
In preparation for their impending wedding, Queen Rotunda forced Mario to exercise vigorously, wanting Mario to become thinner for the wedding, as she simply continued to eat. Eventually, Luigi, Toad and Princess Toadstool escaped Queen Rotunda's dungeon and Queen Rotunda, upon learning this, sent her Beezo guards to find and capture them as she began to set-up her wedding to Mario.
As her Beezo minions searched for Luigi, Toad and Princess Toadstool, Queen Rotunda finally began her wedding to the hesitant Mario. It was only when he was being dragged down the aisle in her castle court yard by Queen Rotunda that Mario noticed Luigi, Toad and Princess Toadstool descending from a nearby tower. Seeing that Luigi had a flask of Passion Potion in his hand, Mario distracted Queen Rotunda by pretending to tie his shoe as Luigi poured his Passion Potion in two glasses on a table directly below him.
Seeing the Passion Potion in the two glasses, Mario grabbed them and proposed a toast to Queen Rotunda and the nearby Prince Pompadour and gave the two the glasses of Passion Potion, which they drank. Drinking the Passion Potion, Queen Rotunda instantly fell in love with Prince Pompadour and vice-versa; after being proposed to by Prince Pompadour, Queen Rotunda accepted his offer of marriage and the two embraced.