Haploid hyphae meet and form a dikaryon: hyphal cells with 2 nuclei.
After growth, the dikaryon sends up mushrooms as fruiting bodies.
The 2 nuclei fuse in some of the mushroom cells, creating a diploid nucleus.
The diploid nuclei undergo meiosis, resulting in new haploid cells.
The haploid meiotic products turn into spores, which blow away and germinate into new hyphae.