Halo: Campaign Evolved adds three new missions to the original Halo: Combat Evolved campaign. These missions form Operation: METEORITE, a prequel story starring the Master Chief and Sergeant Avery Johnson.
The original Combat Evolved campaign contains ten missions and follows humanity’s arrival at Installation 04, the discovery of the Flood, and the effort to stop the ring from being activated. Campaign Evolved preserves that central adventure while expanding the experience with a separate three-mission arc set before those events.
Operation: METEORITE takes place in 2551, about one year before Combat Evolved. Master Chief and Johnson infiltrate a Covenant vessel connected to the glassed human colony of Promise, undertaking a clandestine mission involving rescued UNSC personnel, Covenant experiments, and the search for intelligence.
The number is easy to confuse with the game’s total mission count: ten remade missions from Combat Evolved plus three new missions makes thirteen campaign missions overall. The three additions are not five, four, or two, and they are part of the game’s campaign content rather than a separate Halo title.