Marathon for iPad is free to download from the App Store.
If you want to play it in HD Mode, though, you'll need to fork out a few quid.
Marathon is Bungie's classic Mac shooter, which launched in 1994, a time when dinosaurs roamed the earth and Halo was a mere twinkle in the Xbox's eye.
The iPad update is the work of Daniel Blezek, who used the Marathon source code, released to the open source community in 1999, to build the game.
Bungie got involved and gave the project its seal of approval. Now, on Bungie Day, the day the famed developer bids farewell to the Halo series, Marathon is given a new lease of life.
There's plenty more going on for Bungie Day. Check Bungie.net for the details.