Sperms known as swimmers are ejaculated into a harsh new world, the vagina, where they must survive even though they do not know why they exist. Lots of sperms die, in other words, "expire" as soon as they enter the female's body. Some last a day or two and the lucky ones can last up to four days. Scout swimmers are sent to figure out why they where were they are, and when they come back with good news. Their great mission begins.