The Key to time is one complete season of adventures for the Doctor, Romana and K9 mark two.
The story kicks off as soon as the Doctor has departed Gallifrey at the end of The invasion of time when the TARDIS is intercepted by the White Guardian.
The Guardian has a task for the Doctor, he is to find all six segments of a device known as The key to time - which can be used to stop the entire universe and correct any errors in time and space.
The Black Guardian is desperate to seize the key for his own purposes and will do anything to acheive this.
When the Doctor returns to the TARDIS after his meeting with the White Guardian his new assistant Romana (Mary Tamm) is waiting and both of them - with the new K9, begin the search for the six segments which are scattered all over the universe.
Yes its a long serial but it is quite entertaining.