Okay, the TARDIS crew arrive in a space museum (hence the title) where they see themselves as exhibits in glass cases. In a fascinating science fiction concept, this is explained by their having turned up before they’ve arrived, which is also after they’ve already been there. So when they first arrive for the second time, they’ve already arrived after themselves before they even turn up. So by getting there both before AND after the second time they turn up first, they IT HURTS! IT HURTS! OWWWW……MY BRAIN!