Warning: Spoilers up to the end of Episode 1 follow.
Single twenty-seven year old Aizawa Azusa is a normal salary girl in Japan. But she works too much and dies from overwork. RIP. Lucky for her, a beautiful goddess takes pity on her and reincarnates her into a new world, filled with fantasy and magic! Isekai activate! Not only that, but the goddess honors her request to be immortal and never age physically. And sets her up with a new house.
Aizawa is reborn in her new world as a cute blonde witch. She decides to change her name from Aizawa Azusa to Azusa Aizawa. Hah!
Azusa is loving her new work-free life. The first creature she stumbles upon during her first visit to the nearby village is - you guessed it - a slime. The slime does no damage to her. And she kills it with ease. Azusa learns that the loot the slime dropped can be traded for money at the local adventurer’s guild, so she decides killing slimes will be an easy way to earn living income.
The show flashes forward… to 300 years in the future! The never-aging Azusa has become quite famous in the town. Besides killing slimes, she’s made medicinal plants that she’s used to treat the townsfolk.
While exchanging slime loot for money at the adventurer’s guild as usual, the guild clerk Natalie asks Azusa to check her stats. Azusa last checked them 300 years prior when she first got there. She didn’t expect much change, but apparently killing about 25 slimes every day for 300 years has made her the strongest adventurer in the world! Her level is maxed out at level 99!
Azusa isn’t really happy to learn this. Being that big of a deal means people will come and hassle her for all kinds of reasons. And hassle is work! Her peaceful, relaxing life would disappear. She tells Natalie to keep it a secret. But a big mouth named Ernst overhears and spreads the word, and now everyone in town (and beyond) knows!
The hassles start disrupting Azusa’s life immediately. First, some adventurers want to test their skills on her. She beats four to a pulp with her wind magic, even when she tries to be as gentle as possible. She’s just too OP. Her next visitor is a giant red dragon named Laika who says she’s the most powerful dragon in the region. The dragon also wants to test her strength. But again, Azusa beats the dragon with ease.
During the fight, Laika accidentally damages Azusa’s home. Laika later takes human girl form, pays for the damage, and asks to become Azusa’s pupil and live with her. Azusa eventually accepts, since she expects Laika to do her chores for her.
Laika is able to rebuild the home and expands it very quickly. But Azusa teaches Laika her first lesson - when night comes, it’s time to rest. And enjoy yourself after a hard day’s work!