In 1571, deep within the jungles of Yucatán, a small Maya tribe is devastated by a deadly smallpox outbreak brought by Spanish colonizers. Entire families die as the survivors desperately search for hope. A shaman eventually discovers a mysterious blue plant hidden within a cave rich with vibranium. Believing it to be a gift from the gods, several villagers consume the herb, only for their bodies to transform. Their skin turns blue, and they become unable to breathe air. Forced to abandon the surface world, the survivors flee into the ocean, where they establish the hidden underwater civilization of Talokan. One of the women who drank the herb gives birth to a child named K’uk’ulkan, who grows into a powerful leader revered as a god by his people.

Years later, after his mother dies, K’uk’ulkan fulfills her final wish to be buried in their ancestral homeland. Leading Talokanil warriors onto the surface, he witnesses Spanish colonizers enslaving and brutalizing Maya people on a plantation. Furious at the suffering inflicted upon his people, he orders the plantation destroyed and massacres the colonizers. During the attack, a Spanish priest calls him “el niño sin amor,” meaning “the child without love.” Embracing the hatred he feels toward the surface world and its cruelty, K’uk’ulkan adopts the name Namor and vows to protect Talokan from outsiders forever.

In 2024, Wakanda faces its own tragedy as Shuri desperately works in her laboratory to create a synthetic Heart-Shaped Herb that could save her dying brother, King T’Challa, from a mysterious illness. Despite warnings from her AI system Griot that the formula is unlikely to succeed, Shuri prepares to use it anyway. Before she can finish, Queen Ramonda enters the lab and quietly reveals that T’Challa has already died. Wakanda mourns its fallen king with an emotional funeral ceremony honoring both the Black Panther and the man himself.

A year later, Wakanda faces growing international pressure as foreign governments attempt to seize vibranium resources, fearing Wakanda’s technological superiority. During a meeting at the United Nations, Ramonda exposes a covert French operation that attempted to steal vibranium from a Wakandan outreach center in Mali, proving Wakanda remains strong despite losing the Black Panther. At the same time, an American deep-sea mining expedition successfully detects vibranium beneath the ocean using an experimental machine. Before the miners can recover it, Talokanil warriors attack the outpost, luring many of the crew into the sea through hypnotic songs while slaughtering the rest. Namor personally destroys the escaping helicopter, ensuring no witnesses survive.

While commemorating the anniversary of T’Challa’s death outside Wakanda, Ramonda and Shuri are unexpectedly approached by Namor himself, who bypasses Wakanda’s defenses without detection. Namor reveals the existence of Talokan and explains that his people possess vibranium beneath the ocean. He warns Wakanda that the surface world’s ability to detect vibranium threatens both nations. Namor demands that Wakanda help him locate and kill the scientist responsible for creating the vibranium detector. Although Ramonda distrusts him, Namor warns that refusing his request could lead to devastating consequences before disappearing back into the ocean.

The Wakandan Tribal Council debates how to respond to the new threat. Some leaders argue they should cooperate with Namor, while M’Baku believes Namor should be eliminated before he becomes more dangerous. Okoye is tasked with locating the scientist responsible for the detector, and she convinces Ramonda to allow Shuri to accompany her. Their investigation leads them to CIA agent Everett Ross, who reluctantly reveals that the machine was created by MIT student Riri Williams. When Shuri and Okoye locate Riri in Cambridge, they discover she has already begun developing advanced armored technology similar to Iron Man’s suits. However, FBI agents and Talokanil warriors soon converge on them, leading to a chaotic chase that ends with Shuri and Riri being captured by Namor’s forces.

Shuri awakens in Talokan and is awed by the hidden underwater civilization, its beauty, and its deeply rooted culture. Namor personally guides her through the city, explaining Talokan’s history and his hatred for the surface world. He attempts to convince Shuri that Wakanda and Talokan should unite against the outside world before foreign nations inevitably wage war against them both. Meanwhile, Ramonda seeks help from Nakia, who has been living in Haiti since leaving Wakanda years earlier. Nakia infiltrates Talokan and rescues Shuri and Riri, killing Talokanil guards during the escape. Enraged by this betrayal, Namor launches a full-scale assault on Wakanda.

Namor and the Talokanil devastate Wakanda with overwhelming force, flooding the capital and destroying much of its defenses. During the invasion, Namor attacks the throne room where Ramonda and Riri are trapped by rising water. Ramonda sacrifices herself to save Riri from drowning, dying in the process. Shuri is consumed by grief and rage over losing her mother after already losing her brother. Seeking vengeance, she finally succeeds in recreating the Heart-Shaped Herb using Namor’s mother’s bracelet. After consuming it, Shuri enters the Ancestral Plane expecting to see her family, but instead encounters Erik Killmonger. He tells her that revenge and anger are driving her just as they once drove him.

Shuri awakens transformed into the new Black Panther and immediately prepares Wakanda for war. She creates upgraded armor for Riri and new Midnight Angel suits for Okoye and Aneka. Realizing Namor weakens when deprived of water, Shuri devises a plan to lure the Talokanil into battle aboard a Wakandan sea vessel. A massive conflict erupts in the middle of the ocean as Wakandan warriors clash with Talokanil forces. During the battle, Shuri isolates Namor aboard a Royal Talon Fighter, overheating the ship to weaken him before both crash onto a nearby desert island. Their brutal duel leaves Shuri wounded after Namor impales her, but she manages to severely burn him using the ship’s exploding energy core.

Standing over a weakened Namor, Shuri prepares to kill him and avenge her family. However, memories of T’Challa and the compassion he embodied stop her from becoming consumed by vengeance. Instead, she spares Namor’s life on the condition that Talokan retreat and cease its attack on Wakanda. Namor accepts, recognizing Wakanda as a future ally rather than an enemy. In the aftermath, Riri returns to MIT, while M’Baku challenges for Wakanda’s throne in Shuri’s absence. Shuri travels to Haiti to finally mourn her brother properly, burning her funeral robes and accepting her grief at last. Nakia then introduces Shuri to a young boy—her and T’Challa’s secret son—who reveals that although his Haitian name is Toussaint, his true name is T’Challa, ensuring the Black Panther’s legacy will continue into the future.