This paper describes a Human-Centered Design process aimed at understanding how Virtual Reality (VR) can assist in post-stroke physical rehabilitation. Based on insights from stroke survivors and healthcare professionals, a serious game prototype is proposed. We focused on upper-limb rehabilitation, which inspired the game narrative and the movements users must perform. The game supports two modes: 1- normal version - users can use any arm to pick a virtual hammer and hit objects; 2- mirror version - converts a traditional approach to VR, providing the illusion that the arm affected by the stroke is moving. These were evaluated through a user study.