The Plans is one of the most important of the Bonus Items. They only occur as a primary or secondary target in the mission mode, they are together with the Kit what you are looking for to complete several of the missions.
The plans are the core of the game together with the kits. You have to find these together with the kits to get newer and better Aircraft and proceed in the game. The plans looks like a plan sheet from a typical airfix model kit, 1/72 scale fighter, attached to a golden wing. The plans are trigger objects that will complete your mission when it is picked up together with the kit. It is together with the kit the most valuable items in the game, it earns you 500 points in the Mission Debriefing and completes a "new aircraft" achievement too in the mission debriefing. That means you have unlocked an extra aircraft to use in the game, both in mission mode and multiplayer mode.
Like the kit, you usually don't find it just lying around. You will have to kill an enemy unit to get hold of it, to complete several of the missions.