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In Greece, young Michael Morbius lives in a hospital because of a rare blood disease. There he meets Lucien, another patient with the same condition, and nicknames him Milo. The two become adoptive brothers under the care of Dr. Emil Nicholas. Michael, a prodigy, earns a scholarship to study medicine in New York. Milo stays behind, but the bond holds.
Twenty-five years later, Michael is a famous scientist who invented artificial blood and turned down the Nobel Prize. He's still sick, on crutches and running out of time. He captures vampire bats in Costa Rica and decides to splice their DNA with his own. Milo, now very rich, pays for it. Michael brings his colleague and girlfriend, Dr. Martine Bancroft, onto a mercenary ship in international waters, out of the law's reach. The test works and heals his body, but turns him into a vampire. Starving, he kills the entire crew. Then, horrified, he wipes the evidence, calls for help and jumps overboard.
Back in New York, Michael discovers strength, speed, gliding flight and a kind of bat radar. He also discovers the thirst. Artificial blood holds it off for shorter and shorter stretches. FBI agents Simon Stroud and Al Rodriguez investigate the ship. Milo notices his brother is cured and demands the same treatment. Michael refuses. Soon after, a nurse turns up drained of blood at the hospital, and Michael is arrested as the suspect.
In jail, Milo visits and Michael gets it: his brother stole the serum, took it and became a vampire. Unlike Michael, Milo loves his new life and kills without guilt. Michael breaks out of prison, hides with Martine and creates an antibody that can kill anyone who has the serum. The plan is to use it on his brother. Milo strikes back at the people Michael loves. He kills Dr. Nicholas and mortally wounds Martine. Michael gets there in time to watch her die and drinks her blood.
In the final showdown, Michael summons a swarm of bats to box Milo in and injects him with the antibody. Milo dies in his brother's arms. Michael, now a wanted man, flies off with the bats and accepts what he has become. Unseen, Martine opens her eyes, and they're red. In the mid-credits scenes, Adrian Toomes, the Vulture, suddenly appears in this world from another universe. He looks up Michael and pitches the idea of teaming up.