Let’s start with a quick recap of what’s ahead. At the end of season two, and for reasons best left to another explanatory article, the Doctor (David Tennant) is forced to leave his love, Rose Tyler (Billie Piper), in a parallel universe with a human copy of himself. Most of Season 3 follows the Doctor and his travels with Martha Jones (Freema Agyeman), a medical student who helps him save a hospital being teleported to the moon by cranky rhino people. Like most of the new companions, Martha falls immediately and deeply in love with the doctor, who doesn’t reciprocate in the slightest. He basically tells Martha that she’s a rebound companion, which only makes her more desperate to win his affections. She always proves herself on her travels, but he never gives her anything more substantial, not even to cement a lasting friendship.
In the three-part Season 3 finale, the Doctor and Martha encounter Captain Jack (John Barrowman) when he accidentally crashes the TARDIS into the end of the universe. The trio then meet Yana (Derek Jacobi), an elderly man who is helping the dying remnants of humanity escape to the stars. But meeting the Doctor unearths hidden memories in Yana, who rediscovers that he is the Master, a Time Lord who defied the Doctor many years ago (before the BBC restarted in 2005). With his mind partially intact – he’s never quite sane, not really – the Master steals the TARDIS, regenerates into a new, younger form, and then leaves the Doctor, Martha, and Jack stranded at the end of the universe. Luckily, Jack has a vortex manipulator (read: handy plot device) to take them home, which is now also the master’s home.