Area is a 4-map series in which a Spartan (or several, depending on how many are playing) is trying to escape death.
The first one, Area 52, is an oasis/desert version of Sidewinder with Longsword fighters flying through the sky and a hangar which houses a lot of dead aliens. If you listen before engaging any enemies, you can hear them having conversations.
The 2nd, Area 53, starts with the Spartan in jail, obviously captured during the events of Area 52, and he must escape through an overhead vent (with the help and encouragement of another prisoner) before the executioner comes. The player must fight the guard (with a pistol) and escape through a vent. On the other side of the vent system, the player ends up in a teleporter room that is dripping with Flood goo - the first sign that something is wrong. The player then goes through the teleporter, and ends up on what appears to be an orbital platform. He falls through a space tunnel and ends up in a forest inhabited by dinosaurs and a couple of snipers. The player then infiltrates another part of the base, and battles several more guards before making it (or rather, being teleported) to a room full of ghostly faces.
The next part, Area 54, continues where Area 53 left off. The Spartan is teleported out of the room with the ghostly faces, and into battle with what appears to be undead skeletons, accompanied by demons. After killing them all, he gets on a train. However, the train, being in bad shape, falls through the floor, and takes him to another space tunnel, which apparently leads to the surface of a planet. He is teleported into another, seemingly abandoned, part of the base. After a moment of exploration, he finds the prisoner that helped him escape in Area 53. The prisoner appears to be in bad shape. He is not in his armor - instead, he is sitting on a table, and appears to have had his legs chopped off below the knees. After a moment, the prisoner is taken over by the demonic forces that have seemingly overrun the base, and attempts to attack the Spartan by throwing a grenade at him. The prisoner is killed by the Spartan, and he continues through the base, fighting countless undead skeletons, demons, and headless zombies (apparently former Spartan guards who were taken over). In the vents, he runs into spiders, and quickly dispatches them. After killing more undead swarms, he comes across a really long vent. Jumping down, he lands in what appears to be a Hell-like environment - and runs into the prisoner again. It is noticed that the prisoner now has his legs back - though covered in reptilian scales, similar to that found on the demons. The "prisoner", obviously taken over by demonic forces, taunts the Spartan, and disappears, leaving his demons to kill the Spartan. The Spartan tracks the now-demonized prisoner through the chambers (which appear to be made of rotten flesh), slaughtering any zombies and demons he comes across - until finally he comes across one demon who, seemingly out-of-breath, has a message for the Spartan: to follow him, because "someone important" is waiting for him. The Spartan follows. Once the demon reaches the destination, it makes one final attempt to kill the Spartan, which fails (or it should fail, if the player is smart enough to put some 8-gauge buckshot in the back of the demon's head as it leads you to the teleporter). The Spartan goes through the teleporter and engages the demonic prisoner in a very difficult boss fight.
After defeating the prisoner (you learn that his name was Eric, AKA Halo_hoger), the player attempts to escape in a lifepod, but is shot down by turrets. Upon coming out of the lifepod, he is ambushed by several robot troopers. He fights his way through a forest until he gets to a warthog garage. Apparently there's a secret tunnel that has a lot of respawning enemies in this area, but this is unconfirmed. There is, however, a teleporter that beams you to the top of the canyon, where you will await extraction.