You're a scientist, and your invention, pulsing, glowing ball, called "Sentinel", absorbs and stores energy from different sources. Now the world face deadly aliens, and you have to use your creation to save the planet. Guardian absorbs energy from each of the RMS force or energy capsules you shoot with Atari light gun. When the power source is not hit, ray blasts from the clock strikes the target with a hiss and a attracting force energy back into their patrol. It's up to you to protect sentinel as it glides across the planets.