Fans have been clamoring for an Overwatch event based entirely in-game for a while. Course, that isn't to say that the real world-based events have sucked, but Blizzard has definitely listened. From now until May 1, the archives have been declassified so players can relive one of the pivotal moments of Overwatch's history: the omnic uprising in King's Row.
In addition to the characteristic new emotes/intros/skins/sprays/etc (which you can check out here at the official Overwatch event site) the Uprising event also introduces a new PvE arcade mode. You play as either Torbjörn, Reinhardt, Tracer, or Mercy (which is great for me, a Mercy main) to liberate King's Row from the rogue Null Sector omnics. It's different from the Halloween event and Junkenstein's Revenge; in this one, you can revive your teammates by physically helping them up (not necessarily with Mercy's ultimate) and it includes multiple mechanics such as territory capture, payload defense, and target elimination. There's also another arcade mode with the same principles, except you can play as any character.
You can relive Tracer's very first mission until May 1, when the archives are once again sealed, so pick out what items you're gunning for and get to it. Play, grind, buy—whatever suits your fancy.