While Hearthstone doesn't have a story per se (the cards used in the game are borrowed from the Warcraft universe), the cinematic short introduces audiences to Ava who is wandering around in the cold winter. That is until a mouse transports her to Hearthstone, the inn where magical beings congregate to play the card game of the same name. When she asks the guests about her whereabouts, they all break into song.
The song is a little confusing. I'm not really sure if they're answering Ava's question or evangelizing the card game they all seem to love, but it's a fun little tune and the animation work is gorgeous. Among the singers are Harth Stonebrew, the innkeeper, Malto the wizard, Lou (who drops Hearthstone director Ben Brode's "Well met" introduction), and a bunch of other colorful characters.
Hearthstone recently released its Knights of the Frozen Throne expansion, which was accompanied by a series of live-action videos featuring the Lich King. This is however the first animated media for the card game.
So yeah Blizzard we get it, you have an animation team that can make magic. Now please produce a series where Ava joins a Hearthstone tournament with her Ratatouille-like, card game playing, magic mouse friend.