The Fourth Doctor:

Sarah Jane is still with the Doctor when he is forced into a regeneration after being critically irradiated on the 'Planet of Spiders'. Sarah is dismayed to be abandoned by the Doctor when he is recalled to Gallifrey.
Harry Sulivan is a naval doctor and joins the Doctor and Sarah Jane when he joins UNIT as a liaison officer. After a number of adventures with the Doctor and Sarah Jane, including encounters with the Cybermen and the Daleks, he returns to normal military operations.
Leela of the Sevateem is the descendant of an ancient starship that had crashed eons before. The Doctor finds that in an off-screen appearance, he was responsible for the twisted society in operation and is 'The Face of Evil'. Leela joined him in the TARDIS in order to have an adventure. When the Doctor returns to Gallifrey in order to counter 'The Invasion of Time', she decides to remain there to marry Andred, Captain of the Guards.
K9 is a robot in the form of a dog (very approximately) and was originally the property of the director of the Bi-Al foundation in Titan Space. When the Doctor is invaded by a parasite, Professor Marius sends cloned miniaturised copies of the Doctor and Leela into the Doctor's body in order to destroy it. The Doctor was forced to destroy the base in order to destroy a scaled up copy of the virus. The Doctor has had a number of K9s - one left with Leela, one went off with Romana and a third was delivered to Sarah Jane.
Lady Romana(dvoratrelundar) originally joined the Doctor in order to search for the Key to Time. When this was completed, she was supposed to return to Gallifrey, but had caught the travel bug from the Doctor and refused to do so. She regenerated and continued her travels with the Doctor. Romana finally decides that she ought to stay and help the time sensitive Tharils free themselves from the slavery they once imposed on others. At some stage she goes back to Gallifrey and was forgiven her tardy return to be eventually elected Lord (OK, Lady) President of the High Council.
Adric is supposedly the descendant of a crashed starliner that had crashed more than four thousand years earlier. They are besieged by local creatures that come from the marshes. When the Doctor and Romana arrive, they determine that the people in the starliner are actually more evolved versions of the marsh creatures. Rather than leaving with the starship, Adric stows away on the TARDIS.
Nyssa is the daughter of 'The Keeper of Traken' whose term is due to end. The wife of his successor is less than happy and arranges for her husband, the Doctor and Adric to be arrested. But she was being manipulated by an evil alien being called Melkur. In the ensuing chaos, Melkur is revealed as the Master attempting a thirteenth regeneration. He sort of manages it by merging with Nyssa's father. With no other option, Nyssa joins the Doctor.
Tegan Jovanka was an air hostess with Quantas who entered the TARDIS, thinking it to be a police box after she and her aunt were attacked by the Master. The Doctor was back on Earth in order to find a police box to use as a template in order to repair the TARDIS's chameleon circuits. They all travel to Logopolis, which is inhabited by a group of pure mathematicians. The Monitor, their leader, reveals the fact that they are keeping the universe working long after it should have collapsed. But the Master is not far behind and in his desire to get the Doctor, destroyed Logopolis. This incarnation of the Doctor died when they transferred the Logopolis program to a Terran radio telescope.

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