Fed on a diet of HG Wells novels and Back to the Future films, that video game developers have used time travel as a game mechanic is not new, but it is often limited or constrained, not amounting to more than scripted events or re-do functions. Hazardous Software's Achron is a new breed which sports the most ambitious implementation of a time travel mechanic to date - true free-form manipulation and resolution, and despite a few slip ups it delivers on some unique gameplay experiences.
True time travel may sound complex, but Achron does an admirable job of easing you into the mechanic through the single-player campaign. Narrative-wise the game is intriguing, putting you in the middle of a multi-layered human and alien war, with each side on the conflict containing internal warring factions. The sudden barrage of acronyms and government agency names may leave you dumbfounded, but text logs and intercepted messages will get you up to speed. The voice acting is a nice touch with, good actors generally outweighing the bad, but there are few really poorly delivered lines and some very awkward dialogue which will pull you out of the experience.
Early in the game you'll discover the "chronoportation" technology, and each subsequent mission will introduce a new function or concept for its use. For example, you'll use your ability to change the past to jump back a few seconds to guide a special operations team into an area without being detected. If you fail, and they are discovered, all you need to do is undo that order and wait for a better time to cross. As you progress, you'll be introduced to more advanced tactics like sending a unit back in time to disable a bomb while simultaneously defending your base in the present by switching back and forth between the two points in time or even "chronocloning" your units by doubling your army with copies of themselves from the past or future. Eventually there is going to be a moment during those first few missions where it finally clicks together for you and you'll start using the system without the game prompting you to do so. I even found that this naturally solved an issue plaguing many real time strategy games, allowing you to take risks with important story driving hero characters that would normally be too valuable to commit to combat for risk of failing the mission on their death. This in turn kind of ruins the cool factor of having these heroes if you are afraid to use them. Not so in Achron, if my hero dies I can easily save them in the past by holding them back a bit or arranging them to get some healing. It was satisfying to use my field commanders to actually lead units not simply as a benchwarmer at for the barracks.
What ties everything together are the timeline and "chronoenergy." You can view any point by clicking the timeline itself which takes you back to that point in time, but you have to use chronoenergy to give orders when not in the present. Certain actions are graphed onto the timeline giving you warning to events occurring. You'll see the spike of combat as your opponent moves a force to attack you in the past, which lets you counter before suddenly your base disappears in the present. Time waves also carry the events forward, so cause and effect is further delayed to give you room to react to a changing battlefield in more than one place in time. Chronoenergy quickly refills over time, but the farther back you are and the more units you try to control the more energy is required. This really helped to keep the games tight given the scope of its applications and frees you up to do some things that have been previously unheard of in the real time strategy genre.