In Asylum , Nyssa shares an adventure with the Fourth Doctor from a time before he met her , leaving the Doctor with the knowledge that he will have to be extremely careful dealing with Nyssa when they eventually meet to avoid changing history . In Asylum , Nyssa shares an adventure with the Fourth Doctor from a time before he met her , the Doctor having been tracking a change in history which has somehow led him to Nyssa 's futuristic Home . This adventure leaves Doctor with the knowledge that he will have to be extremely careful dealing with Nyssa when they eventually meet to avoid changing history while ensuring she survives to meet him in his past .