Karfel

Karfel was a planet inhabited by humanoids known as Karfelons. (TV: Timelash) It was located in the same galactic cluster as Earth. (PROSE: Speed of Flight)

Astronomical data
Karfel had diplomatic relationships with the planet of the Bandrils. A council in the Citadel governed life on Karfel. (TV: Timelash)

Somewhere on the planet was a leisure garden with singing plants. (TV: Death of the Doctor) As well as the singing flower, there was a flower with purple petals that could shoot acid. (TV: Timelash)

History
The Third Doctor first visited the planet with Jo Grant and Mike Yates about 20,000 years in the future, at the height of its civilisation. (TV: Timelash, PROSE: Speed of Flight) Jo also visited a leisure garden with singing plants. (TV: Death of the Doctor)

During this visit, the Doctor saved the entire planet and reported to the local authorities the "unethical" experiments of a scientist named Megelen on a local life form called Morloxes. His actions were recorded in Karfelon history books and a local artist painted a mural of the Third Doctor in the Citadel. The Doctor also gave a locket containing a photo of Jo to a Karfelon who handed it down to his granddaughter Katz. Before leaving, the Doctor promised he would some day return to Karfel. (TV: Timelash)

The Sixth Doctor's TARDIS became trapped by a kontron tunnel and landed on Karfel once again. By this time, Megelen, as the Borad, now ruled the planet as a dictator with the aid of androids. He had also ordered that the story of the Doctor's visit be ordered from the planet's history books. The Borad created the kontron tunnel, which he called the Timelash, and used it as a means to exile his enemies.

With his companion Peri and a young H. G. Wells, the Doctor freed Karfel of the Borad and averted a war with the Bandrils. (TV: Timelash).

Behind the Scenes
In The Doctor: His Lives and Times, there is a space-time postcard to Mike Yates from Jo that goes into detail about the trip she and the Third Doctor took to Karfel. Jo tells him that Professor Megelen is "ever so dashing and took a fancy to me but it seems he's gone a little potty and started experimenting on the local cattle (horrid things called Morlox)". Jo's letter suggests Megelen was making the Morlox attack people, and was a "lovesick mad scientist" who made advances toward her, but the Doctor saved the day with the help of the Bandrils. It also suggests Professor Megelen was an old friend of the Doctor's before turning evil.