Spoiler : Planet Earth existed in a place where there was a strange discontinuity in the space time continuium. This means 2 things 1. Earth's problem is : elsewhere things happen which are most probable, but on Earth the improbable happens more. The improbable is more probable in reality. 2. There is another Earth in a parallel universe, so it wasn't destroyed by the Vogons when Earth was destroyed for a hyperspace bypass. A space ship has been sent to protect it, but in an accident the memories of the beings are destroyed, so they know they have to observe and wait, but they don't know what they are looking out for.
Arthur finally gets back to Earth, knowing he is safe as it phrophesised he will die at a place called SMB. But the Vogon destroyer ship comes in behind the sun from the protectors, so they don't see it. Actually the leader who has forgotten his mission has pointed the guns away from it anyway cos of what he read in his horoscope. Arthur and all are at a club he realises too late this is SMB. So Arthur had got back, but this time he is destroyed with the Earth. The story has come full circle. Every trace of Earth is gone !
Lessons - Trillian learns the life lesson : Never Go back for your handbag. Then for the most important thing in her life she finds the rule doesn't apply. ie. rules sometimes fail.
Arthur loves Earth and puts up with a lot of shit to get back, even abandons a life where he is treated as a king - but then when he finally gets back it's destroyed.
DA should have written it so tha the Vogons were seen by the protectors, but then the head read his horoscope and pointed the guns away anyway.