Is there a stowaway aboard the TARDIS? No, it's just Romana, who has regenerated off-screen, for no apparent reason, and chosen to do so into an identical double of Princess Astra. (One might think that she'd had enough of being a princess's exact double after the incident on Tara, but clearly our Romana is made of stronger stuff than that.) This story was a turning point for the Daleks -- every Dalek story for the next decade was more of a Davros story with some Daleks in it than an actual Dalek story. Watch it here