In a way, it's a bit of an overstatement to call the three stories included in this set a trilogy, for each is very distinct in both style and substance. Thankfully so, since variety is the spice of life. Linking the three ever so loosely though is the introduction of Turlough and his Faustian bargain with the Black Guardian, a cosmic being with a grudge against the Doctor. In a way, saddling the Doctor with a "rubbish" companion (well before Adam in 2005's "Dalek" & "The Long Game") is an inventively risky idea, and having a thoroughly unpleasant and devious character among the Tardis crew adds a suspenseful twist to the stories on the one hand, while on the other it often devolves into Turlough repetitively stepping not quite but almost out of earshot and hissing into a glowing crystal for instructions from his erstwhile Mephistopheles. Be that as it may, something beyond the bounds of the tried and true is laudably being attempted here. The trilogy as a whole also happens to transpire roughly during the middle of Peter Davison's tenure as the Fifth Doctor, a moment when his approach to the character seems to have become comfortably established while still retaining some of its initial freshness. On that score, then, these three stories amply demonstrate in both their significant strengths and minor pitfalls a good deal of what "Doctor Who" was capable of in 1983.
"Mawdryn Undead" is a particularly strong story that unusually makes the most of the potential for time paradox in the show's premise and even more unusually features an antagonist whose ambition is not to conquer nor destroy but to die. The reappearance of the Brigadier along with other references to the show's history are rather skillfully woven into the plot, although the resolution is a bit of a let-down, taking place not through the Doctor's own cleverness nor even the bravery of his companions past and present but limply through a credulity-straining coincidence. Likewise straining one's credulity in "Terminus" is the idea that a load of starship fuel could set off a chain reaction destroying not just a planet, a solar system, or even a galaxy, but the entire universe (!). One can't help but suspect that the writer's concept of cosmology and its scale is a bit uninformed. That said, the story deftly balances the grim imagery of drudgery and disease with resonant mythological motifs while confronting the contradictions between the profit motive and medical care in a way that coincidentally seems unusually relevant more than two decades later. Finally, "Enlightenment" is the real gem of this trilogy. There is something indescribably eerie about it. And yet it's also just an incredibly well-written science fiction tale juxtaposing impeccable expertise in historical detail with an utterly surreal context. The tone can pitch dramatically in range from hauntingly mysterious, boisterously campy, firmly moralistic, and strangely romantic--and yet the tale as a whole, while marred a bit here and there by some overacting, holds together wonderfully and verges on the profound without losing its sense of fun and adventure.
Come to think of it, then, on a deeper level certain rather weighty themes do in fact run through these three stories after all: temptation and redemption, death and regeneration, time and eternity, order and chaos, not to mention the unsuitability of dead birds as headgear.