  We present a rigorous theoretical model for the optical interaction between a
nitrogen-vacancy (NV) centre in diamond and a plasmonic metal nanoparticle
(MNP), accompanied by a computationally efficient procedure to solve the
evolution. The proposed model enables us to successfully explain existing
optical emission measurements of NV centres both in the presence and absence of
a metal nanoparticle. We show that the NV-plasmon interaction provides a
versatile new avenue to enhance and control the optical emission of an NV
centre. Changes to the MNP type and size, NV-MNP centre separation, submerging
medium permittivity, and NV orientation with respect to the MNP surface can be
exploited to improve a plethora of NV centre based nanodevices.
