Roy Mustang's Observation Diary is a bonus chapter to the Fullmetal Alchemist manga, released along the Volume 6 in Japan. Differently from other bonus chapters, this one is not made out of images, but written exactly like a Journal, telling the story as if Havoc, Breda, Fuery and Falman were reporting what they see. Images are occasional, as to illustrate the story.
Second Lt. Jean Havoc is appointed by the upper brass of the Amestrian Military to conduct that year's efficiency evaluation regarding the work of his superior officer, Colonel Roy Mustang. Havoc decides that the best way to evaluate the Colonel's job is to produce a observation diary, where he would take notes about the Colonel's conduct during working hours. He asks for the help of his fellow military, Kain Fuery, Heymans Breda and Vato Falman to write down notes about Colonel Mustang's behaviour when he's not around.
The four of them then take turns in writing down Mustang's activities, and Jean always seems to try to enhance Mustang's faults, while the others try to reason the Colonel is actually working hard and "doing his best" with the amount of work and responsibility that lies on him. A series of jokes ensue in these notes, as the Colonel is swamped with paperwork and doing it at the last minute, then spending ridiculous amounts of time cleaning the office for inspection, participating in meetings and looking bored (but trying his best to hide it), trying to bond with Lt.Hawkeye's dog, Black Hayate, and failing miserably, among others.
Lt. Havoc finishes his report, saying the Colonel might look a good employee, but has several faults at his work that get in the way of his efficiency. He later receives a letter from the upper brass stating that, although he has made a terrific job in evaluating Colonel Mustang, he has been observed during this time by Major Sargent Kain Fuery and was accused of having slacked on his own job and pursued Mustang wherever he went. He was adverted that he shouldn't forget his own job during his evaluation and was, therefore, punished with the subtraction of half of that month's paycheck.
Colonel Mustang, on the other hand, was considered to be a loyal and dedicated officer, who did everything in his power (including stepping out of his confort zone) to make the Military achieve its meanings and received a bonus.