My first reaction to this excellent game was "I'm not sure what's happening but it's fast". And that's my considered opinion as well. The reason I'm not sure what's happening is that neither the cassette insert nor the program instructions give you any idea of the scenario.
Having said that, it doesn't take a lot of K to realise that the amazingly animated, ferociously fast objects swirling and swooping down from the top of the screen are nasties. And any arcade novice should be able to realise that the laser base at the bottom of the screen can be moved from side to side, avoiding bombs and replying in kind with lasers.
No, there's no problem with the game itself, a really fast example of ultimate Electron arcade action. It's just that the instructions are a bit of a puzzle.
You can have the sound on or off, decide whether you want the one or two player game, keyboard or joystick. You can pick the start screen, whether you want automatic fire or not or even set the reload speed. But you have to discover for yourself what such things as automatic fire and the reload speed actually mean.
Not that you have much time to spend trying to find out, the game is too good for that.
Zalaga is entrancing. The graphics are superb, fast and effective. The control keys are easy to use and (remarkably) well explained and the game concept simple but appealing. The idea of aliens dropping from the sky may be old fashioned but in Zalaga it reaches the state of the art.
It's a superb action game, flawed only by the lack of explanation. Even so, it's thoroughly recommended.