With the original series, you can generally jump in at any time. Well, it depends on how they air it. It was originally half hour episodes then at some point they went to hour episodes. A single story might span 4 or more episodes. So, in those cases, obviously you want to start watching at the beginning of a story. Some stations, such as my local PBS station, would air an entire story in one night. Saturday night’s were typically Doctor Who night. It’d start at 10:00 and run until midnight or longer. But, that way you got to see the whole story.

With the new series, the stories are generally an hour and standalone, but there have been some 2 and 3 parters. But, you really want to jump in at the start of a season, because the new series has made use of season-long story arcs. So, there’s generally a little something in each episode that will slowly build to the finale. This past season, they used stuff from the previous three seasons as well. So, that was pretty cool.

There’s only one Doctor, but he has different incarnations. You see, if he dies, he can regenerate. He can regenerate 12 times, so there would be a total of 13 different Doctors. Same character, different incarnations. Each has a different personality.

You could, of course, start with the first episode.