One characteristic of Christopher H. Bidmead's writing for Doctor Who that did manage to sneak its way into Frontios was Bidmead's love of abusing the TARDIS. After jettisoning over 25% of the TARDIS during the events of Logopolis and Castrovalva, Bidmead finally dispatches with the TARDIS entirely by apparently blowing it up at the end of Episode One.
The trouble is that while the TARDIS is obviously not destroyed (there wouldn't be much of a show if it was), it might have helped suspend the disbelief if Tegan, Turlough, and (especially) The Doctor would have reacted to the news that their only means of transportation away from a doomed colony had just evaporated by treating it as something more than a mild inconvenience. Someone who is so used to traveling unfettered by the constraints of time and space wouldn't have dismissed the TARDIS's destruction so willingly.
Turlough has some nice scenes with Norna in this episode as the two form a pseudo-Doctor/companion partnership, although Turlough does prove he is completely useless with girls by barking orders at Norna when trying to open the entrance to the tunnel. He did the same thing with Preston in Warriors of the Deep, and often was uncivil to Tegan during their early days together. Has no one taught him anything about charm?
Mark Strickson usually is quite good as Turlough, but his ranting and drooling scenes that start in this episode are a little too over the top, even for Strickson. It doesn't help when director Ron Jones cuts to an extremely cheesy closeup of his crazed expression just after The Doctor stops him from bolting out of the tunnel.