Relativistic quantum field theory is extended so that it can be used as a basis for a qualitative study of nuclear resonance reactions. The general resonance formulae are derived in a relativistic form, and the interference between neighbouring levels is investigated. It is assumed that resonance arises from the formation of a compound nucleus which subsequently disintegrates. The essential features of resonance and level width are, in the first instance, derived from a simple resonating model. This shows that, while resonance arises directly from the Feynman propagator in lowest approximation, the level widths come from considering an infinite series of Feynman-Dyson diagrams; these can be represented by an integral equation. In considering a general nuclear resonance reaction it is necessary to use compound propagators, which were introduced by the author in the first paper of this series. The general form of the compound propagator is obtained in stable approximation, and the integral equation is derived which allows for the possibility of disintegration. The solution of this equation leads to the relativistic formulae for a general resonance reaction.