“The Commune sent your group to find a stash rumoured to be in the warehouse district. It should have been safer with the heavy rainfall, and you did get the stash, but that’s when everything went to hell! There was an explosion. A trap, or maybe just bad luck. The dead poured in and your group scattered. Escaping into the flooded utility tunnels, you splashed your way through the darkness with the echoes of the infected close behind you. Finally, you find a ladder to the surface, and the hatch opens without a fight…”

Note: This can be a dark setting with nasty things happening to people. Talk to your DM before starting to establish boundaries.