Successor is a very loose term in this case, the events in Shadow of Chernobyl take place after those in Clear Sky. Or, in other words, Clear Sky is the prequel. Something which is made abundantly clear by including "Prologue" on the box cover of the game. The game places you in the fictional setting of a desolate Chernobyl (aka "The Zone") in 2011. Desolate, safe for a group of S.T.A.L.K.E.R.S. Actually, a group... make that multiple groups.