Heart of TARDIS by Dave Stone

Heart of TARDIS features the second Doctor and the fourth Doctor as as they must unravel a unimaginable terror set to rip the universe apart.

In the small American town of Lychburg something strange is going on. The towns citizens are being killed in the most horrible and brutal ways and the police don't have any idea who is behind the attacks. The only suspects are a mysterious and sinister stranger, who calls himself the Doctor, and his young companions Jamie and Victoria.

Meanwhile The Fourth Doctor and Romana have been summoned by the Gallifreyan High Council. A force has been unleashed into the space/time continuum... a force so unimaginably terrible that it is set to rip the universe itself apart and plunge it into primal, screaming chaos from which nothing will survive.

Of course, since something of this nature happens every other day of the week, the Doctor's really far more interested in finding out what's happened to a close personal friend, who seems to have vanished under mysterious circumstances. And quite right, too. The fate of a universe plunging into fetid and unending chaos can look out for itself for a change...