故事開始在一個被三道高牆保護的人類世界。百年來，人們相信牆外已沒有人類，只有以人類為食的恐怖巨人。主角艾連·葉卡從小嚮往牆外的自由世界，直到某天，60公尺高的「超大型巨人」突然出現，踢破最外層的瑪利亞之牆，隨後出現的「盔甲巨人」也攻破城門。在混亂中，艾連親眼目睹母親被巨人吞食。

五年後，艾連與青梅竹馬米卡莎、阿爾敏加入調查兵團，立志消滅所有巨人。在一次任務中，艾連為救阿爾敏被巨人吞食，卻意外覺醒變成巨人能力，反過來消滅其他巨人。這讓軍方陷入恐慌，經過審判後決定利用他的能力奪回瑪利亞之牆。

在奪還作戰中，調查兵團付出慘重代價，終於抵達艾連家的地下室，發現驚人真相：牆外不僅有人類，還存在科技更先進的瑪雷國。原來艾連等人都是被瑪雷流放的艾爾迪亞人後裔，巨人力量源自始祖尤彌爾的傳說。更震驚的是，阿爾敏發現從小一起訓練的同伴萊納和貝爾托特竟是當年破牆的「盔甲巨人」與「超大型巨人」。

隨著真相揭開，故事舞台轉向海外。瑪雷國為維持霸權，將艾爾迪亞人變成巨人作為武器。已成戰士的萊納等人奉命奪回「始祖巨人」艾連。經過激烈戰鬥，艾連接觸到擁有王室血統的希絲特莉亞，覺醒始祖之力，得知更黑暗的歷史：2000年前，始祖尤彌爾為初代弗里茨王獻出心臟，開啟巨人詛咒。

最終季中，獲得始祖之力的艾連發動「地鳴」，釋放牆內數千萬超大型巨人踏平世界。昔日的同伴被迫組成小隊阻止他，米卡莎在最後關頭親手斬殺艾連，終結地鳴。巨人之力隨之消失，但世界對艾爾迪亞人的仇恨仍未平息。故事在滄海桑田後結束，留給讀者關於戰爭與和平的永恆思考。

The story begins in a world where humanity lives within three concentric walls, built to protect them from the terrifying Titans—giant humanoid creatures that devour humans. For a hundred years, people believed that humanity had perished outside the walls, and that the Titans roamed freely beyond. The protagonist, Eren Yeager, dreams of the freedom that lies beyond the walls. However, one day, a 60-meter-tall Colossal Titan suddenly appears and kicks a hole through Wall Maria, the outermost wall. Soon after, the Armored Titan breaches the gate, allowing the Titans to invade. In the chaos, Eren witnesses his mother being devoured by a Titan.

Five years later, Eren joins the Survey Corps alongside his childhood friends Mikasa Ackerman and Armin Arlert, vowing to destroy all Titans. During a mission, Eren sacrifices himself to save Armin and is swallowed by a Titan—only to awaken within the Titan's body and use its power to fight back against the others. This shocks the military. After a trial, they decide to utilize his abilities to reclaim Wall Maria.

The operation to take back Wall Maria comes at a heavy cost. Eventually, the Survey Corps reaches the basement of Eren’s childhood home and uncovers a shocking truth: humanity does exist outside the walls, and the world is far more technologically advanced. They learn about the nation of Marley, a military superpower that has exiled Eldians, the descendants of Ymir Fritz—the original Titan. The Titans’ powers originate from her legend. Even more shocking is Armin’s discovery that their trusted comrades Reiner and Bertolt are actually the Armored and Colossal Titans who destroyed the wall five years prior.

As the truth unfolds, the stage shifts beyond the walls. The Marleyan government uses Titans as weapons to maintain global dominance, transforming Eldians into mindless Titans. Reiner and his fellow warriors are sent on a mission to retrieve the Founding Titan, which resides in Eren. After intense battles, Eren makes contact with Historia, a royal-blooded descendant, awakening the Founding Titan’s true power and learning a darker secret: 2,000 years ago, Ymir Fritz gave her life and power to King Fritz, beginning the curse of the Titans.

In the final season, Eren—now wielding the power of the Founding Titan—initiates the Rumbling, unleashing millions of Colossal Titans hidden within the walls to flatten the world. His former friends are forced to unite in a desperate attempt to stop him. In the end, Mikasa kills Eren to put an end to the destruction. With Eren’s death, the Titans’ power vanishes forever, but hatred toward the Eldian people lingers.

The story concludes after many years have passed, leaving readers with deep reflections on war, freedom, and the fragile hope for peace.