Louis Marks is definitely among my all-time favorite Doctor Who writers. I count "Day of the Daleks", "Planet of Evil" and "The Masque of Mandragora" as three absolute classics and upon watching "Planet of Giants" once again I found that while it doesn't quite reach the sheer excellence of his other stories, it is a wonderfully fun, massively entertaining story with spectacular design and convincing effects (the fly that Barbara suddenly finds in front of her, for instance). It is short, effective, and again, just plain fun.
This story marks the composing debut for Dudley Simpson on Doctor Who and the beginning of what all fans know is a very, very long tenure as composer for him. "Planet of Giants" features many of Simpson's fine points as composer, his ability to enhance drama by tastefully allowing the visuals to tell the story, his ability to add a light touch to scenes of adventure and action, and his wonderful ability to enhance scenes that may otherwise drag. The direction, which is mostly by Mervyn Pinfield is confident and quite good throughout, helping the episode move at a fast pace.
Back to Louis Marks. In short, "Planet of Giants" is as good as it ever wanted to be. Marks clearly had no aspirations of making this a comedy classic, a sweeping epic adventure, an intensely dramatic tale, a scary horror effort, or an action-packed story. "Planet of Giants" in concept and in execution is a cheeky, fun little diversion and nothing more. While some fans may hold this as a criticism I refuse to do so simply because the script is technically very competent- it has an assured sense of pacing, the dialogue and attempts at humor work, and the story almost bizarrely achieves a sense of wonder despite being a contemporary Earth story with a plot device that is not all too original. If I had the opportunity to see this story as a child I am certain it would have been one of, if not my absolute favorite Doctor Who story. It's fun, whimsical, and thoroughly entertaining.
Episode 1: 8/10, Episode 2: 8/10, Episode 3: 8/10.
Average: 8/10