We study holomorphic discs with boundary on a Lagrangian submanifold L in a Kaehler manifold admitting a Hamiltonian action of a group K which has L as an orbit. We prove various transversality and classification results for such discs which we then apply to the case of a particular Lagrangian in CP3 first noticed by Chiang. We prove that this Lagrangian has non-vanishing Floer cohomology if and only if the coefficient ring has characteristic 5, in which case it generates the split-closed derived Fukaya category as a triangulated category.