What happened to Marissa Marcel? That's the central crux of the splendid video-game-slash-interactive-movie(s), Immortality. It's a rare treat, anchored by an exceptional lead performance from the actor Manon Gage, in which you attempt to solve the mystery of what happened to a promising actress who starred in three movies, none of which ever saw the light of day, and then promptly disappeared.

Your task here is to peruse mountains of recorded footage — both scenes from the aforementioned movies as well as behind-the-scenes archives — and unlock further scenes by pausing the film and clicking on points of interest as you see them. If you click on a wine glass, you'll unlock another scene that includes a wine glass, and so the ones most difficult to unlock are those tied to the more unique or specific items.

You're never quite sure what will be unlocked next. So, you'll perhaps see a climactic scene from one of the three movies, and then next witness a table reading where actors discuss the motivations of a character for an entirely different film. Because the scenes are unlocked in no particular order, it's utterly bewildering for much of the time you'll spend with it, but it's a testament to the quality of the writing, the performances, and the reverence and authenticity of the movies here that we remained enthralled all the same.

It took us seven hours to reach the point where the credits started rolling, and if you're anything like us then when they do you may well be somewhat bamboozled. It's easy to miss important details. But even after the ending you can continue scrubbing footage, and we recommend you do. There's a plethora of secrets hidden in this wonderful game, and finding them is required to understand the truth. And that's the joy of the whole thing.

It sounds ridiculous, but as we got sucked into Immortality's tangled web, there were times we forgot the movies we were watching weren't real. It's an astonishing accomplishment, really, and one we can't recommend highly enough.