Twelve is set during a day at a public swimming pool. Tilde announces to Alva that she has just had her first period. This sets off a mounting sense of insecurity in Alva, which only increases when she notices her crush paying Tilde more attention.

A young gang member is found dead, and the Beck group's investigation indicates that a drug network is behind it. Klas Fredén has been monitoring the network for a long time, and the Beck group - led by Alex Beijer - is most graciously allowed to work with the murder investigation in parallel. She will soon end up on a collision course with Fredén. Thus, Martin Beck, who is now head of department, is also involved in the case.