Barbara and her Menoptera friends hide in the Temple of Light, while the Doctor and Vicki escape after taking control of a Zarbi.
"Invasion" is an improvement on the last two episodes. Bill Strutton isn't good at pacing his episodes at all, but he writes his characters beautifully. Prapillus' soliloquy at the beginning is wonderfully written, and performed beautifully by Jolyon Booth.
I'm glad William Hartnell and Maureen O'Brien get more to do this episode. I liked how Vicki named their captured Zarbi "Zumbo"!
Ian is written terribly by Strutton, however. Ian's has nothing to do the past two episodes; it's not a wonder that William Russell decided to leave the series while making this episode. However, Strutton writes the Optera beautifully. Rather like Prapillus, Strutton gives Hetra very lyrical dialogue.
Richard Martin's direction seems to be getting slightly better. He has some quite nifty camera angles throughout.
This episode is quite scary: Nemini sacrifices herself plugging up an acid stream, while the cliffhanger of the Doctor and Vicki getting sprayed in a web must have frightened the kids watching this in 1965!