Private detective Ferdinand Zankl is arrested by the Passau police on suspicion of murder. He is suspected of having killed his ex-girlfriend, an animal shelter manager.

Competition rider Nina Becker and movie star Niklas Tremsaal are a happy couple without a marriage certificate. Marrying is not an issue for the two - until the little niece India holds cheap plastic jewelry on Nina's finger for an engagement ring and so gets an avalanche rolling, which can not be stopped. - "Marry makes me nervous" is a joke-staged romantic comedy. Suzan Anbeh plays marrying Nina, who has to choose between her fiancé and her childhood sweetheart.

At first, friendgroup TKKG have no idea that something is amiss when “Jugend forscht” winner Kevin behaves strangely during the presentation of his “Mind Machine” - they blame his nervousness and his unexpected termination of the speech on relationship stress with his girlfriend Nadine. But when Nadine disappears without a trace shortly afterwards, like three other children before her, the amateur detectives know that they have to help the police with their investigation.