In the future year of 1992 (the story was originally published in 1989), aliens have come to Earth looking for buried treasure and enslaved humanity in the process. They capture a man in London named "Leapy" who's trying to escape to safety. They're just about to execute him, when that familiar blue box appears, and out jumps the Doctor, who wastes no time in bravely, heroically . . . singing "Happy Birthday." Yes, as is so often the case with The Doctor, particularly on the original series, he's wound up in entirely the wrong time and place. He was aiming for the birthday party of a friend named Bonjaxx and has even brought along a present. (NOTE: Apparently, in a later comic, he actually makes it. "Party Animals," published in 1991, takes place at Bonjanxx's party and features the Seventh Doctor meeting a later regeneration of himself.) But, as long as he's in town, he might as well save the day. Or get himself captured. In the Doctor's world, the two are often indistinguishable.
The plot of this story is very similar to "The Dalek Invasion of Earth," a popular First Doctor story arc, and one of the earliest appearances of the Doctor's longtime enemy. But, "Invaders from Gantac" has a much lighter tone and is even a bit silly. You wouldn't think the story of a dystopian alien apocalypse could be so much fun, but it is. The aliens are sort of similar in personality to the Daleks but are less hate-driven and more bureaucratic. Every action they take is given a numbered order, or sub-order, or sub-sub-order, and occasionally arguments arise as to which order supercedes which others.
It's interesting in these stories so far to see the Doctor without his companions. We tend to define the Doctor as much by the people who travel with him as by his own personality traits. But, in the three stories presented in the two Doctor Who Classics issues so far, he always shows up alone and simply teams up with whomever he finds when he arrives--usually accidentally. Whatever the creative reasoning for this may be, it does allow us to become more deeply entrenched in the world in which the Doctor has landed, by giving him a guide who knows that world.
Fans of Doctor Who will definitely enjoy "Invaders from Gantac," even if they're not familiar with the Seventh Doctor's exploits, specifically. Those who aren't specifically fans of Doctor Who will probably enjoy it too, as long as they know at least the basic premise of the show. It's well-drawn, well-written, funny, and a lot of fun.