During Blizzcon 2017, Blizzard announced that Starcraft 2 is going Free To Play. Wings of Liberty, the Terrain campaign, will be made free alongside the co-op missions up to level 5 and online ranked multiplayer.
Despite you lacking the other expansion, online multiplayer will give new players access to the new units added into the game. With the ranked play, "All you have to do to unlock it permanently is earn 10 First Wins of the Day in Unranked or Versus A.I. play—our way to preserve the quality and integrity of the ranked experience.", says Blizzard.
for those that already own Wings of Liberty, Blizzard will offer the second expansion Heart of the Swarm free instead. Starcraft 2 will be available to all starting November 14.
For more details on the Starcraft 2 Free to play, head on over to its official site.