Made a video for it here.
Basically, after scanning the CCTV Junction Box in the supermarket, the player is attacked by zombies after going outside. If the player dies before reaching the safe house, they'll respawn and the game will want them to go back inside the supermarket. When the player does this, the mission indicator directs them outside, then back inside, etc. The player is unable to progress past this point.
It's a bit disappointing. I wasn't expecting this from one of the major Wii U launch games.