The Quintessential Phase
The Quintessential Phase has one sub-plot of Zaphod attempting to reach Zarniwoop (which did occur in Fit the Twelfth, but that version was dismissed during the Tertiary Phase as Zaphod having a "psychotic episode" - another version of events occurs here, in an attempt to interconnect all five series). Zarniwoop has been merged with the character Vann Harl from Mostly Harmless. This radio series also sees the return of the characters of Max Quordlepleen, Thor, and Zarquon (who all appeared in Fit the Fifth at Milliways), and also Mr. Prosser, from Fit the First. None of these characters appear in the book version of Mostly Harmless.
The start of some of the plot threads from Mostly Harmless were introduced in the Quandary Phase (though they did not appear in So Long, and Thanks for All the Fish) - notably the Vogons discovering that Earth had re-appeared and resolving to destroy all versions of it, Arthur having sold his DNA, a mention that Trillian now has a child and is now a reporter. Also the final episode saw the introduction of an alternate version of Trillian, still known as Tricia McMillan, who is identical to the original Trillian except for being blonde and American. This alternate McMillan is played by Sandra Dickinson, who played Trillian in the television series.
Read more about this topic: The Hitchhiker's Guide To The Galaxy Tertiary To Quintessential Phases