Complex sentence: In this case, if the catcher's direct throw retires a runner the play stands, if not, the ball is dead and all runners must return to the bases they had most recently touched (before the catcher's throw).
Split into simpler sentences: In this case, if the catcher's direct throw retires a runner the play stands. If not, the ball is dead and all runners must return to the bases they had most recently touched (before the catcher's throw).