Beginning in the middle of the third season, some episodes are primarily based on land.

The third week's episode is based on a military theme.

The episode was based on actual reports of conflict between the two nations.

Each episode was based on the theme of a specific mood.

This episode was based on a remark David once made during a conversation.

The whole episode and its fallout really wasn't based on what one witness heard or didn't hear.

Each episode is based on the life of a letter sender.

The first episode is based on the first book, written and set in 1958.

Most episodes are not based on the books but are, in fact, new stories.

Many episodes were based on real life crime cases.