While the Doctor is still recovering from his regeneration, this episode begins with a set of highly intelligent twins working on computations. They are not surprised to discover their mother has gone out again, but are rather disappointed when their father goes out. Once they are left alone, a strange visitor stops by claiming to look for their father. They persuade him to leave, but he places hypnotic disks on their arms so they lose their memory. He has both of them grab his arms as he activates a teleportation ring and they disappear.
Inside the Tardis, the Doctor is having trouble stabilizing after his regeneration. He doesn't feel quite like himself and finds himself very paranoid. Stabilizing for a bit, he attempts a change of cloths, to which Peri find hideous. But the Doctor is also having fits of rage. During one such fit, he accuses Peri of being a spy and attempts to choke the life out of her. Peri succeeds in fending off the Doctor when she uses a mirror to blind him. The Doctor is barely able to apologize when again he destabilizes and claims Peri is a coward when she does not want to search for a survivor from a crashed ship they discover.
Peri thinks no one could have survived the ship wreck, so when the Doctor finds a survivor he degrades Peri for wanting to give up so quickly. They take the survivor to the Tardis, then go to investigate a building they see off in the distance. When they are captured, the Doctor hides behind Peri in terror and claims it was Peri's fault they are there. Taken inside, the Doctor meets Azmael, a fellow renegade Timelord and friend. The Doctor is shocked to discover Azmael barely remembers the Doctor and leaves him and Peri locked in a building which one of Azmael's lackeys rigs to detonate. Azmael seems to be involved in a radical plot, which is very unlike his nature.
Under pressure, the Doctor is able to use his wits to escape the exploding room. He safely teleports both of them back into the Tardis where a now conscious Hugo is waiting for answers. He's been investigating the disappearance of the twins and thinks the Doctor is involved. Peri is able to convince him they are innocent. Somewhat reluctantly, they team up to find who kidnapped the twins and why. The Doctor realizes his old chum Azmael is involved, but not sure to what degree. Initially he's furious, thinking Azmael knew about the explosion.
The real villain is a creature called Mestor whose been pressuring Azmael into helping. Azmael has agreed, thinking he's protecting the people of Jaconda. But Mestor's plans are more shocking than Azmael has bargained for. Mestor has a fully stocked armada of larva which can only hatch in intense heat. The Doctor shows Azmael that Mestor plans to explode the sun surrounding Jaconda in hopes of spreading the larva throughout the entire galaxy. Now knowing this, Azmael agrees to help the Doctor thwart Mestor's plans.
The problem is, Mestor is too well protected, as demonstrated when the Doctor tries to destroy him. Mestor's mental abilities put up a strong defense. To further prove his point, he places his own conscious inside Azmael's. This time it's Mestor who is tricked, as Azmael has distracted the gastropod long enough for the Doctor to attack his body once more. This time the attack works, but Azmael is killed int he process, having used up all his regenerations. As he dies, he tells the Doctor he does indeed remember the last time the two met.
Bonus Photo: The Doctor selects his outfit.