This episode also gives Danny a new wife.
In the first three seasons, he had been married to a woman named Margaret, played by actress Jean Hagen. Ms. Hagan became dissatisfied and left the show when her contract expired after the 3rd season. So at the start of the 4th season, Danny was suddenly portrayed as a widow struggling to raise his two kids alone.
They introduced his future wife, a nurse named Kathy (played by actress Marjorie Lord), in the last few episodes of the 4th season, and got them engaged just three episodes after they met (in the last episode of the 4th season). In this first episode of the 5th season, Danny & Kathy have just gotten married and are on their honeymoon in Las Vegas.
This was the first episode broadcast on CBS.
This episode marks the sudden appearance of Angela Cartwright as Linda, Kathy's daughter and Danny's stepdaughter,.
The character of Linda had only been seen once before, just two episodes previously, near the end of the 4th season. And at the time, Linda was played by another child actress, named Lelani Sorenson. She was eight years old, and had relatively light-colored hair (like Marjorie Lord, her TV mother) tied back in two pigtails. She was shown to be a very shy girl at first, who then quickly becomes an ill-behaved brat who wreaked havoc on the Williams household.
That Linda made only that one appearance and then vanished.
The next time we see the character of Linda, in this episode, she is a completely different girl, played by Angela Cartwright. Linda is suddenly three years younger (just five years old) and significantly smaller than the original Linda, she now has dark brown hair tied in a single ponytail, and is a perfectly-behaved young lady.
Apparently, they decided to recast the role of Linda with a new actress at the start of the 5th season (at the same time that they had Danny marry Kathy). They probably assumed no one would notice given the six-month break between seasons, and the fact that Linda had previously only made a single appearance.
Angela Cartwright replaced Lelani Sorenson as the character of Linda beginning in this episode.