The end of your story must be the final climactic battle between the hero and villain. This doesn’t necessarily have to be a massive action sequence with explosions and special effects. Instead, the climactic battle must be a do-or-die moment for the hero. If the hero fails, his or her life will be wrecked forever.
The simplest way to wreck the hero’s life is to threaten physical death. That’s how action films work in the climactic battle such as “Die Hard” where the hero must defeat the villain or risk losing his wife forever.
Sometimes the hero doesn’t face physical death but emotional death where the hero’s life would be changed forever. In “The Karate Kid,” the hero doesn’t risk getting killed, but if he loses, he’ll risk having the villain forever dominate over him. By standing up and winning against the villain, the hero actually wins over the villain so we know the hero will never have to worry about the villain again.
In “Sully,” the hero is the pilot who landed a plane in the Hudson River without causing a single death. Yet at the end, he has to face an investigative committee that has the power to determine the cause of the crash was pilot error, which would ruin the hero’s reputation and career forever.
If the hero loses, he won’t risk physical death but death of his reputation, which is just as terrible. As a result, the climactic battle is still tense because the hero’s battle is a do-or-die moment where the hero either wins or loses completely.
The hero in “The Edge of Seventeen” is a 17-year old girl who hates her life and is fighting with her brother, mother, and best friend. For the climactic battle, the hero doesn’t face physical death but emotional death in staying forever estranged from the people around her. In this climactic battle, the hero repairs her relationship with her mother, brother, best friend, and ultimately finds happiness as well.
The climactic battle at the end can be a huge battle to the death (“Die Hard”), a physical battle of some kind (“Rocky”) without the risk of death, or emotional death (“The Edge of Seventeen”). The main key of the climactic battle is that this is the last fight where the hero either wins or loses for good. Because this final battle is the last chance for the hero, it creates additional tension and suspense, which makes the ending more memorable.