The first Nova Covert Ops mission pack for StarCraft 2 arrives 30th March and you will be able to play it without owning the main game. All you need is the free Starter Edition installed.
The Nova Covert Ops mission packs are £6 a pop, and there are three of them, each containing three missions. They're storied missions that follow - to begin with - a Ghost operative called Nova. They take place in the Koprulu sector and involve beating the Defenders of Man and helping Emperor Mengsk. You'll need to infiltrate and uncover the baddies using all of your super sneaky abilities.
The other two Nova Covert Ops mission packs haven't been dated but will be out before December, according to Battle.net, where you can buy all three packs as part of a £12 bundle.