Play a single mission on its own — no campaign, no saved progress.
Start Playing Right Away drops you into a random mission with random characters — start immediately.
Your characters are either assigned at random or chosen by you. Either way, the app tells you exactly how many heart tokens to use and which abilities are unlocked (and which aren't) for the mission.
You cannot unlock more abilities during a mission. Start another mission and everything resets — you may get different characters, different amounts of health, and a different set of unlocked abilities.
Whenever a character gains XP, place the token on the space showing their current Roll Modifier, covering it — from then on they use the next-highest modifier. This repeats until they reach their maximum modifier; after that, any further XP is simply discarded. XP has no other use in this mode.