You play as either Ana as the Predator, or a team of 4 Survivors (who can be made up of a combination of Roadhog, Hanzo, Junkrat, Mei, and Moira). There's a 15 minute time limit and the aim is to get to three points; a total wipe of the opposing team is one point.
Roadhog, and Mei have had their self heal taken away, and Moira can only heal with orb.
Ana is invisible, and becomes visible when she shoots or uses an ability. She can either kill or sleep the enemy team to earn victory.
Sleep and Death are permanent for the round. Sleeping players can be woken by teammates or by damage from Ana.
There's also a bunch of other little surprises in there in terms of abilities and hero balancing.