The central premise of "Frieren: Beyond Journey's End" is built on the aftermath of the hero's party defeating the Demon King. The story's first major "twist" is the death of the hero Himmel from old age in the first episode, which triggers Frieren's journey of regret. Throughout the first season, Frieren's past as "Frieren the Slayer" is revealed. She is an incredibly powerful mage who has lived for over a thousand years and was instrumental in developing magic specifically to kill demons. She consciously suppresses her mana as a deceptive tactic, a technique taught by her own master, the legendary mage Flamme. This is demonstrated in her fight against Aura the Guillotine, where Frieren reveals she has accumulated vastly more mana over her lifetime, allowing her to turn Aura's soul-weighing magic against her and force her to commit suicide.
The latter half of the first season adapts the First-Class Mage Exam arc. Frieren, Fern, and several other mages participate in a series of tests to gain passage to the treacherous northern lands. During the exam, we meet Serie, a mythical elven mage from the past and Flamme's master, making her Frieren's grand-master. Serie holds a deep-seated disdain for Frieren's way of life and her magic, leading her to fail Frieren in the final stage of the exam. However, Serie sees immense potential in Fern and passes her, officially making her a First-Class Mage. The season ends with Frieren, Fern, and Stark finally cleared to travel north, with the ultimate goal of reaching Aureole—a place at the former Demon King's castle said to be where the souls of the dead reside—still far ahead. Himmel's constant presence in flashbacks reveals his unrequited love for Frieren, a fact she only slowly begins to comprehend throughout her journey, adding a layer of tragic romance to the narrative.