The Doctor has regenerated, having sacrificed his fifth persona to save Peri's life. But things are not going well... On this occasion the process of regeneration is by no means smooth, for the even-tempered, good-humoured Fifth Doctor has given way to a rather disturbed and unsettled successor. In a particularly irascible moment the new doctor comes close to committing a shocking crime. Overwhelmed with guilt of his violent behaviour, the repentant Time Lord decides to become a hermit...
Source · Open Library