SOS is the last of the three monochromatic arcade games released by Namco, in early 1980; the first two were Navalone and Kaitei Takara Sagashi (and the latter was originally created by K. K. Tokki as a prototype). It's a fixed shooter in which the player must use a 2-way joystick to direct a fighter plane known as a "Shinryaku" across the bottom of the screen while enemy planes fly down towards it from the top of the screen - and you can press the Firing Button to make the Shinryaku fire bullets from its nose at the enemy planes to destroy them for 10 points apiece. You can only fire one bullet at a time, and if you miss, you have to wait for the bullet to go off the top of the screen before you can try again - and occasionally, a flashing arrow will appear and point to either side of the screen, while an "SOS" signal is being heard in Morse Code. If you can manage to make it to that side of the screen before it disappears, you'll receive 30 extra points, and the number of enemy planes that have bypassed your Shinryaku will decrease by 9 (but if it is under 9, it will have no effect); if the number of enemy planes that have bypassed the Shinryaku amounts to 100, the game will immediately end regardless of how many lives you have remaining, which can be annoying for all players who have not lost a single life. It's also worth noting that when you fire a shot at an enemy plane to kill it, the resulting explosion can kill your Shinryaku as well - and this can also be annoying for the players who can't get away in time.
One of the reasons this game is infamous, is because of the "coffee breaks" it takes after every 2000 points, depicting a young woman above the text of "X000 TEN, COFFEE BREAK" (X being any even number and ten, written in Kanji as 点, meaning "points" in Japanese); on the first two times, she will be wearing a bikini, but if the arcade owner has set the "nudity" dip switch to "on", she will be topless after 6000 points, and completely naked after 10000 points. As a result, this could be considered the first game from Namco that is not suitable for all ages, eight years before their Japan-exclusive beat 'em up of Splatterhouse was released - and they would not feature nudity in any of their subsequent games, at least not until another of their Japan-exclusive titles, Dancing Eyes, was released in 1996.