A passenger spaceliner and a small cargo ship collide while entering Hyperspace. The Doctor and Romana offer their help in separating the two craft, and soon realise that Vraxoin, the most lethal and addictive drug in existence, is being smuggled aboard the liner. The Doctor is also concerned when he finds a naturalist called Tryst using an unstable Continuous Event Transmuter, which stores entire ecosystems on laser crystals. This device starts releasing Mandrels, a dangerous lifeform from the planet Eden.