The penetration of enveloped viruses into target cells requires the fusion of the lipid envelope of their virions with the host lipid membrane though a stepwise and highly sophisticated process. However, the intermediate steps in this process have seldom been visualized due to their rarity and rapidity. Here, using cryo-electron tomography, TIRF microscopy, and cell membrane-derived vesicles called blebs, Ward et al. visualize intermediates of the HIV-cell membrane fusion process and demonstrate how Serinc proteins prevent full fusion by interfering with this process.