S.O.S. stands for Save Our Souls, the international distress signal. You may already know this, but if you don't the learned intro on the cassette blurb will tell you. In this case the S.O.S. is coming from stranded astronauts on a planet surface and it's your job to fly a shuttle down to the ground and rescue them. From this you will already have gathered that what we have here is another 'Rescue' or 'Lunar Lander' type of game.
In this version the animated mothership is quite large. Your shuttle is equipped with a thruster and laser fire which shoots a beam out in the direction of vertical movement. It's needed because there are asteroids in plenty on the way down and firing aliens on the way back up.
Each astronaut, there are six, must be rescued one at a time by landing on one of three platforms on the planet's surface.
With each landing the platforms are reduced in usefulness, thus reducing your choice of landing sites as each rescue is carried out.
Progressing to harder screens, there are more aliens to dodge and pulsing stars appear at the edges of the landing sites, making the final approach more difficult. Points are scored for space debris shot to pieces and for the rescue of the stranded astronauts.