Quote Originally Posted by Silver_Persian
Can somebody tell me why I can't get into mewtwos cave?

And also how do I get onto the islands after the third one
Speak to the guy who's repearing the R/S Machine then go to Moltres Montain, on half way to the montain you'll see on the left two Team Rocket Members, beat them, find the Ruby Plate and bring it to the guy, he'll give you another ticket for the Islands 1 - 7.

To get into Mewtwos Cave, find the two Plates.