You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Doctor_Who

Context:
The companion figure – generally a human – has been a constant feature in Doctor Who since the programme's inception in 1963. One of the roles of the companion is to remind the Doctor of his "moral duty". The Doctor's first companions seen on screen were his granddaughter Susan Foreman (Carole Ann Ford) and her teachers Barbara Wright (Jacqueline Hill) and Ian Chesterton (William Russell). These characters were intended to act as audience surrogates, through which the audience would discover information about the Doctor who was to act as a mysterious father figure. The only story from the original series in which the Doctor travels alone is The Deadly Assassin. Notable companions from the earlier series included Romana (Mary Tamm and Lalla Ward), a Time Lady; Sarah Jane Smith (Elisabeth Sladen); and Jo Grant (Katy Manning). Dramatically, these characters provide a figure with whom the audience can identify, and serve to further the story by requesting exposition from the Doctor and manufacturing peril for the Doctor to resolve. The Doctor regularly gains new companions and loses old ones; sometimes they return home or find new causes — or loves — on worlds they have visited. Some have died during the course of the series. Companions are usually human, or humanoid aliens.

Question:
What is the only story of the original series where Doctor Who travels alone?

The Deadly Assassin