An accident with higher-dimensional aliens turns Spock fully human just before a family ritual with T'Pring, and Chapel helps him hide it.
A shuttle accident brings Spock and Chapel into contact with the Kerkhovians, a higher-dimensional species. In healing Chapel they accidentally make Spock entirely human, just before an important engagement ritual with T'Pring's family. With the crew's help he tries to pass as Vulcan while a cure is sought.
Chapel persuades the Kerkhovians to restore him by admitting her feelings for Spock. Spock completes the ritual, exposes the prejudice of T'Pring's mother T'Pril, and T'Pring, hurt that he kept his condition from her, asks for a break. Spock then admits his feelings to Chapel, and they kiss.