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T'Challa is dying of a mysterious illness. Shuri tries to recreate the heart-shaped herb, destroyed by Killmonger, in her lab, but runs out of time, and the king dies. Wakanda says goodbye with a funeral full of white robes and music.
A year later, Queen Ramonda rules and stands up to countries that want vibranium by any means, even by force. Shuri buries herself in work to avoid her grief and believes the world no longer needs a Black Panther. In the Atlantic, an American team uses a machine that can detect vibranium and finds a deposit on the ocean floor. Blue-skinned warriors led by Namor kill them all. Namor slips into Wakanda and gives Ramonda and Shuri an ultimatum: hand over the scientist who built the detector, or he attacks. He blames Wakanda for showing the world that vibranium exists.
Shuri and Okoye find out the scientist is Riri Williams, a 19-year-old MIT student. They try to protect her, but Namor's people beat Okoye and take Shuri and Riri to the bottom of the sea. Ramonda strips Okoye of her rank as general and calls on Nakia, who lives in Haiti. In Talokan, Namor shows Shuri his kingdom: a Maya people who ate a vibranium-laced plant in the 16th century and began living underwater. He proposes an alliance against the surface world. If Wakanda refuses, it will be the first target. Nakia sneaks into Talokan and rescues Shuri and Riri, but kills two guards during the escape. Namor strikes back with an attack on Wakanda. Ramonda drowns while saving Riri, and Namor vows to return with his whole army.
After her mother's funeral, Shuri uses a sample of Talokan's plant to finally recreate the heart-shaped herb. She drinks it and, on the Ancestral Plane, it isn't her mother who appears but Killmonger, goading her toward revenge. Shuri becomes the new Black Panther and orders an attack on Talokan, despite M'Baku's pleas for peace. Ayo takes over the Dora Milaje, Okoye gets new armor and Riri builds her own iron suit.
The Wakandans lure Namor to the surface with a ship. In the battle, Shuri traps Namor in an aircraft that crashes on a deserted beach. She gains the upper hand but remembers her mother and chooses peace. Namor accepts surrender and an alliance. Riri goes back to MIT, M'Baku decides to challenge for the throne and Shuri plants new herbs. In Haiti, she finally burns her mourning clothes. In the mid-credits scene, Nakia introduces the son she had with T'Challa and raised in secret. The boy says his Wakandan name is T'Challa. The future of the throne is left open.