From the introduction:
Clowns. We've all seen them... riding their unicycles, blowing up balloons at kid's parties, piling in garish hordes from tiny cars, waddling around in bulbous red shoes, passed out in a dumpster stinking of booze and regret. For what do these martyrs of mirth sacrifice themselves? The answer is they do it for us. Not just to fill our need for laughter and merriment, but because they defy the doom of mankind! Clowns are man's only salvation against... Vampires.
You've seen Vampires too; you just didn't know it. Maybe they were killing the mood at a party, ruining a great punchline, layering red tape onto a business, or just plain old-fashioned sucking the blood from the living. Contrary to myth they do much of this in broad daylight, their only fear of the sun being that it might expose them to mockery. They drain humor from the world because they fear someone might dare to laugh at them. You know the type.
Vampires are right to be afraid, as their immortality depends on their dignity. Clowns must destroy a Vampire in a hilarious way for it to stick. This war has gone on for ages, and unfortunately the old “Gags” like garlic and crosses barely get a chuckle nowadays. Sure, folks thought being impaled on a stake with a brick in your mouth was the acme of comedy once, but today's Clown plays for a tough crowd. Fortunately, Vampires fall into the “Straight man” role so eagerly that it isn't impossible to set up a killer Gag.
...and do it you must, or the world will be drained of everything that makes it worth living. Ridding the world of life-sucking undead is your sacred duty as a member of Clown Helsing!