(This a review of all four parts of The Ark in Space)
Despite some dire costume and make up effects The Ark in Space remains essential Doctor Who for various reasons. It was the first classic Tom Baker story and set the tone for the early part of his long reign in the TARDIS in a way that his pilot story, Robot, failed to do. It is also a very original and well written story that seems to have influenced some of the most important science fiction in the decades that followed.
Its plot, in which a band of humans emerge from cryogenic freezing to discover their mission is being thwarted by a deadly alien creature, predates that of classic horror film Alien by five years and the human to insect transformation sequences were echoed twelve years later in David Cronenberg's disturbingly memorable Fly remake. The intelligent tactics of the alien species in these episodes, such as cutting the power to thwart the Doctor's rescue efforts, are also used in James Cameron's Aliens.
Sadly, as with much 70s Who some unsuccessful special effects may hinder the enjoyment of modern non-Who fans but if you're willing to ignore some bad rubber costumes you'll find Ark in Space to be a well written, memorable slice of British sci fi that helped marked the beginning of the most popular era in Doctor Who history.