The film is set in a snow-bound, concrete community outside Stockholm sometime in the seventies. Brezhnev is still General Secretary in the USSR.
It's the disturbingly touching story of a bullied misfit kid called Oskar and his neighbour Eli, who is also 12, but admits to having been that way for some time. Both are lonely and Oskar discovers that his need for an intimate pal is going to override all other considerations.
Eli lives with a creepy middle-aged man who others could easily mistake for her parent, but who functions essentially as her vassal, tasked with collecting her blood rations. His ineptitude in this respect permits us to see how this little girl — who walks barefoot in the snow and smells vaguely of putrefaction — is able to take on burly, blue-collar types considerably larger than herself.
The title derives from the clause in vampire lore which states that one has to formally invite them into your home. There's also an amusing scene playing on the attitude of cats to traveling Transylvanians.