The Doctor has regenerated but it hasn't gone well. He's mentally unraveling and needs a place to rest beyond outside influences. Unfortunately the Master has laid a trap for the TARDIS that denies the Doctor undisturbed recovery time.

The city of Castrovalva and its untroubled people would seen an ideal place for the Doctor to recover, but a dire warning from Adric, held captive by The Master, makes it seen less so.