Marathon is a first person shooter in which you play the security officer on board an interstellar colony ship, which unfortunately has just been boarded by alien forces determined to wipe you out. Unless, that is, you can defeat them first.
The game is an open source recreation of Bungie's classic Mac shooter from 1994, and there's definitely a Doom-like retro feel to its opening stages, as you run around narrow corridors blasting the pixelated alien hordes.
Marathon isn't just about destruction, though. There is some real depth to the gameplay, with plenty of puzzles to solve, and a substantial story to figure out. As well as capable multiplayer options to keep you entertained.
What's more, Marathon is just the beginning. The game was followed by Marathon 2: Durandal and Marathon Infinity, and the Aleph One engine also supports a great deal of community content, from new levels to complete new games.
Retro graphics mean this won't be for everyone, but if you like Doom-style shooters or loved the original Marathon then there's plenty of fun to be had here.