Virgin Missing Adventures
Following the success of the New Adventures series based on the Seventh Doctor and his companions from Survival onwards, Virgin Books began a series Virgin Missing Adventures. These featured the adventures of the first six incarnations of the Doctor.
The first Missing Adventure served as a sequel (and also prequel, from the Doctor's point of view) to the New Adventure novel Blood Harvest, in an obvious ploy to get readers to buy both books. The plot of the Missing Adventure, Cold Fusion took the form of a direct crossover between the two series in the form of the Fifth and Seventh Doctors appearing in the same novel together, along with the latter's companions Roz and Chris Cwej.