Retro/Grade has an interesting concept, presenting a Rhythm game in the style of a side-scrolling space shooter...played in reverse. The first level, is therefore the end. At first, you get to put the final few shots into the final boss, and the credits begin to roll. Suddenly, everything starts to go backward. From then on you have to undo all Rick's shots and re-avoiding every enemy attack.
At first, the concept seems crazy and takes a while to get used to. If you keep in mind it is a rhythm game, it does make things easier. Everything is happening to the beat of the music, and you know that you must be in the correct lane to undo the shots that are coming right to left. You can virtually ignore the bullets coming from left to right on the easier difficulties, but may need to pay more attention on the harder ones where more bullets will be on-screen, and a larger focus is placed on timing.
The playing field can be seen as Rock Band/Guitar Hero's note highway scrolling horizontally. Where there would be notes, there are now weapon projectiles, and you move vertically to meet them with a well-timed press of a button. As well as the standard projectiles, there are missiles where you tap repeatedly, lasers where you hold the button, and sections where you are just dodging lasers, black holes, or moving between streams of bullets.
Like Rock Band/Guitar Hero, the number of lanes varies by difficulty, starting with only two for the beginner level and going up to five for the tougher difficulties. Every lane is colour-coded the same way too, with the colour of every projectile showing you which lane to be in.
Failing to undo any of Rick's shots or getting hit causes damage to the space-time continuum. If it's destroyed, the game is over. However, you can take advantage of your Retro/Fuel that allows you to fast forward time to undo your mistakes. You lose any multiplier you may have built up, but it gives you the opportunity to hit the notes, save your health, and gain a few more points.
The visuals have all sorts of bright colours and fancy lighting effects and you can argue that it's completely overdone. The moving backgrounds, flashing lights, and amount of projectiles can be hard on your eyes but you can tweak the visuals in the graphics options. This is pretty much required if you want to go for a high score (well, a lower one); using the over-thruster power can be blinding which is pretty much guaranteed to make you miss a shot.
The music is brilliant, although sadly; there's only 10 tracks. Due to the many difficulty levels, and the fact you will want to beat your scores, you can still get hours of fun out of them. In the Challenge mode, there's a map with branching paths which offer all sorts of twists. This can be to achieve a certain score, achieve a certain multiplier, playing a higher tempo, making no mistakes, only achieving perfect timing and even visual tweaks like different colours or playing with a zoomed-in camera. Completing certain challenges unlocks different extras, such as new ships, music for the jukebox, and artwork. All these challenges are based on the same 10 levels from the main campaign though, so even while they're great songs, there's definitely a point where people won't be able to play any more.
You can play the game with either the keyboard, a controller, or even a guitar peripheral! The Shooter and Rhythm styles have separate leaderboards which adds to the longevity. Personally, I've found it to be the greatest Indie game that I've played so far. It takes a new innovative spin on the rhythm formula, the graphics are impressive, the sound is brilliant, and there's hours of fun to be had.