Can the Doctor and his friends save the day? Or is Earth doomed? Does anyone who has ever seen a sci-fi series not know the answer to that question?
The solution to stop the fungus, (SPOILER) it can be killed by rain, is a little too simplistic for my taste. Even if the Martians control the weather, can they really hold off water falling from the sky forever, a natural part of the planet's process? And if they do, what kind of widespread, devastating effects might that have on the globe as a whole? Perhaps the Martians don't need such liquid to survive, and don't care about other life. Maybe the fungus will make permanent changes, and it won't matter if it's destroyed after a certain period of time. But the way that this conclusion is presented doesn't seem to hold up to scrutiny if one stops to analyze the larger implications.
The T-Mat seems to be pretty plainly borrowed from the American Doctor Who contemporary series, Star Trek, which began using the transporter in 1966, over two years before "The Seeds of Death" aired.
That being said, "The Seeds of Death" is a solid story. It's hard to go wrong when delivering a clear villain against which to root. This is a typical hero tale, with the good guys fighting against evil. They suffer losses while doing so, but the major characters live to fight another day, and the enemy is repelled. The environmental overtones, which may not have been intended, but are present when viewed with modern sensibilities, as well as the distasteful caste system of Martian society, further separates "us" from "them," providing a serial as black and white as the film of the day.
So perhaps "The Seeds of Death" isn't as complex and deep as other Doctor Who adventures, but it's a fine mix of the usual sci-fi adventure ingredients. A lot can be said for entertaining popcorn entertainment, especially when it is presented with the quality of the Doctor Who brand.