Joe's granpa has an expensive new computer with the very latest computer mouse—a Megamouse. When Joe and his friend Kelly sneak into Granpa's room to play games on his computer, they discover that Megamouse can see and hear and communicate with them. But Megamouse is not content to be confined to Granpa's room. With the help of Granpa's pet rat, Cleo, Megamouse decides to explore the world outside. But there he is in great danger from the ferocious bulldog, Hogarth, and his sinister owner. Megamouse maybe a fantastic aid on the computer screen, but how will he cope with the real world?