A new era begins for Doctor Who and things begin to unravel quickly for the newly regenerated Doctor. Despite the preperations, the regeneration is failing and the Doctor is effectively out of commission and must rely on the rest of the Tardis crew to help him through this delicate time. The first problem is with Adric, who has been captured by the Master and now exists as a projection. The Masters goal is to trick the Doctor into a trap within a trap. Though Adric seems to be helping the Doctor, he actually sets the Tardis on a specific course as ordered by the Master.
After Adric suddenly disappears from the Tardis, Tegan and Nyssa help the Doctor find the Zero Room - A portion of the Tardis set aside to help Timelords heal from troublesome regenerations. The girls manager to find the Zero Room just in time, but a new problem is emerging. Neither girl knows how to fly the Tardis and it's on a collision course with 'event one', the greatest explosion ever; the Big Bang! Fortunately (at least they think so), they find the Tardis Index File. Using the I.F. they decipher where they are headed and how to avoid it, but the Tardis doesn't have enough power to escape the pull of event one. Things get worse as the Tardis begins to heat up the closer it gets to the Big Bang.
In his delicate state, Tegan opts to not tell the Doctor about Adric missing though know they still need his help in order to escape from their current predicament. Tegan suggests finding enough power to escape the gravitational pull and the Doctor gives them a way. By ejecting 1/4 of the Tardis, enough thrust will be achieved to finally escape the Masters plan. The Doctor gives the girls enough information to do the task on their own as he collapses from exhaustion. Tegan is worried because the 1/4 of the Tardis ejected is random and she's afraid the control room could be one ejected. Nyssa points out if Tegan doesn't do as the Doctor ordered they are going to die anyway. Reluctantly Tegan complies at the last second. The Master watching from afar laughs gleefully, thinking the Doctor and Tardis have been destroyed. But he's also disappointed he didn't get to spring the second part of his trap. To his horror and delight, he learns from Adric the Doctor did escape.
Free of event one, the girls help the Doctor back to the Zero Room, but it's one of the rooms jettisoned. Nyssa helps the Doctor build a special Zero Cabinet out of what's left of the Zero Room as Tegan heads back to the console room to search for a tranquil place to land the Tardis. The I.F. points her to Castrovalva, a calm and peaceful place which will help the Doctor complete his regeneration. After a rough landing, the girls carry the Doctor to the city but lose him on the way as they try and climb up to the city, situated on a high cliff. After they lose the Doctor, they are captured themselves. But the Castrovalvans are a friendly sort and agree to help the Doctor who is now suffering slightly from amnesia, and in search of the Doctor.
The Portreeve is the wise man of the city and takes an immediate interest in the Doctor. But as the Doctor starts to remember who he is, things take another turn for the worse as the city of Castrovalva begins to affect his mind as it folds into itself. Things don't add up till he realizes the Portreeve is actually the Master who's using Adric's mental equations to create Castrovalva and all the people. Though created by the Master, he doesn't control them and they give their lives to rescue Adric and help the Tardis crew escape. Only then does the Doctor tell Tegan she didn't really fly the Tardis because the Index File was another projection from the Master, so he effectively piloted the Tardis to Castrovalva.
Next: A Death In The Family