For the problem of detecting a subspace signal in the presence of signal mismatch, we propose a two-stage detector, referred to as the AESD. The AESD is constructed by cascading two detectors, namely, the adaptive energy detector (AED) and the adaptive subspace detector (ASD). The AESD offers great flexibility in controlling the detection performance for mismatched subspace signals, since the AED is very robust, while the ASD is very selective. We derive the analytical expressions for the probabilities of detection and false alarm, which are verified by Monte Carlo simulations.