Valve has detailed a significant new update for Team Fortress 2, which consists of both free and premium content.

The part of the update you have to pay for is the Gun Mettle campaign. This is a roughly three-month event during which you're assigned various contracts--described as "skill-based challenges"--that offer various rewards upon completion. Example contracts include killing an airborne enemy using a Soldier secondary weapon, living through 1000 damage in one life as the Heavy, or killing an enemy with a reflected projectile using the Pyro.

The rewards aren't just typical weapons: Each is one-of-a-kind. Valve says that every weapon has a "uniquely placed paint job" that will make it at least "a little different" from those that other players will get their hands on. There are various levels of rarity, but no matter which one you end up with, you won't have a competitive advantage; what sets these weapons apart is purely a matter of looks.

Participating in the campaign requires the purchase of a $6 campaign pass. Buying this puts a Contract Coin in your inventory, a non-tradable item that tracks your progress and certain stats over the course of the campaign. This is more fully detailed in an FAQ on the TF2 website.

In addition to all of this, there's a good deal of new content for those who don't wish to pay. Valve is releasing a new map, Powerhouse, which it says is the first three-point Control Point map. In addition to this, a trio of new community-developed maps are coming to the game, with the makers of these getting a cut from sales of campaign passes.

There are also new taunts, achievements, and the ability to exchange your weapon for one dropped by deceased players (provided it can be wielded by your current class). Previously, the only time you could alter your equipped weapons is when you respawn.

Alongside all of the new content, a number of gameplay changes have been made. This includes tweaks to many of the game's classes (perhaps most notably, the Spy and Engineer) and the addition of a new feature that allows you to inspect the look of your weapon when wielding one of the new decorated items from the Gun Mettle campaign.

The Gun Mettle update, as all of this is called, is due out today on Steam. More details are available on the TF2 website, where you'll also find a new comic.