Friends play many kinds of games with each other: cops and robbers, checkers, tag. The best of friends will make up their own games.
Todd and Kevin's friendship is built on such a game. However, the rules and premise are far from the typical childhood games. A dispute amongst the two splits them into very different directions: one sees the game for the cruel act that it is, while the other decides it must move to the next level.
Imagine No Country for Old Men crossed with Lord of the Flies, or even imagine if Johnny the Homicidal Maniac were once a little kid. There you have a Friendly Game.