The standard philosophical analysis of counterfactual conditionals the Lewis-Stalnaker analysis - analyzes the truth-conditions of counterfactuals in terms of nearby possible worlds. This paper demonstrates that this analysis is false. Section 1 shows that it is a serious epistemic and metaphysical possibility that our "world" is a massive computer simulation, and that if the Lewis-Stalnaker analysis of counterfactuals is correct, then it should extend seamlessly to the case that our world is a computer simulation, in the form of a possible-simulation semantics. Section 2 then shows that a Lewis-Stalnaker-style possible-simulation semantics clearly fails as an analysis of the truth-conditions of counterfactuals in two types of simulated worlds: Humean Simulations and Necessitarian simulations. Section 3 then considers and answers several objections. Finally, Section 4 draws several skeptical lessons about counterfactuals.