A teenage girl discovers a talking cat only to find out she must return him to his owners in 1970s Stockholm.

For color:
Where in 1970s Stockholm does this take place? A quaint cottage, the grandiose buildings of town square [Old Town], or an orphanage?
Is the teen lonely or a misfit? What’s her arc?
Does the cat have a unique perspective of the times? Trying to paint a picture of this cat.
Is there a - cat must be returned BEFORE…? If not, finding a family is good enough.

I agree. Let us know what happens if he doesn't return the cat. Bad idea: could the teenage girl be mute and the cat speaks for her? I don't think we need 1970s or Stockholm. It does read like a time travel movie.

To give your story dramatic stakes there has to be a pressing reason why she needs the cat.