Simon Bar-Sinister invents the Valentine Vault- a device which turns humans into pretty paper valentines.  He tests it out on a cross section of people  to make sure it works.   Then he kidnaps Sweet Polly Purebred & uses her to lure Underdog into the vault.  Soon Underdog is just a valentine.  Simon decides to keep him for use as a dartboard.  However, it turns out the Vault only flattens the subject and sticks them on paper.  Underdog's Super Energy Pill gives him the strength to bust out of his prison.  He tracks Simon and his henchmen down at City Hall (where Simon is trying to take over the City) and turns Simon into a valentine.