The TARDIS arrives on the space liner Empress but someone on board the liner is smuggling the dangerously addictive drug vraxoin. It's up to Four, Romana and K-9 to save the day! Unfortunately an airlock malfunctions and everybody is sucked out into space and chokes to death. Possibly.
Not 'alf bad. It's often been cited as the worst, but it is, in reality, even objectively speaking, miles better than Underworld and Destiny of the Daleks. To be frank, it's bashed solely because many Doctor Who fans are stoners who don't want to admit to themselves that there is difference between pot and coke, and therefore refuse to admit that not all drugs are as harmless as marijuana. Such fans are will deny the fact that a lot of the less organic drugs can do permanent damage to your vascular system, or worse, can make you mistake your girlfriend for an alien spy and strangle her (but really, some of them can almost literally make you do that; if I had a nickle for every rape story that could be attributed to cocaine...). Obviously Spectrox has similar chemical properties to cocaine. Which makes more sense than it probably should.
To be fair, at any rate, you sometimes need drugs to enjoy Doctor Who as a whole. You just have to draw a line somewhere.