While staying with his weird great-aunt Kathryn, Evan visits a funky old toy store and buys a dusty can of monster blood. It's fun to play with at first. And Evan's dog, Trigger, likes it so much, he eats some! But then Evan notices something weird about the green, slimy stuff-it seems to be growing. And growing. And growing. And all that growing has given the monster blood a monstrous appetite...