Summary: For an agent to be able to rely on a plan, he must know both that he is physically capable of carrying out the physical actions involved, and that he knows enough to carry out the plan. In this paper, we advance and discuss new definitions of `knowing enough to carry out a plan', for the case of a single agent carrying out a sequence of primitive actions one at a time. We consider both determinate and indeterminate plans. We show how these definitions can be expressed in a formal logic, using a situation calculus model of time and a possible worlds model of knowledge. The definitions strictly subsume previous theories for the single-agent case without concurrent actions. We illustrate the power of the definition by showing that it supports results of the following kinds: 1) Positive verification: showing that a plan is feasible. 2) Negative verification: showing that a plan is infeasible. 3. Monotonicity: the more an agent knows, the more plans are executable. 4. Reduction for omniscient agent: for an omniscient agent, a plan is epistemically feasible if and only if it is physically feasible. 5) Simple recursive rules that are sufficient conditions for the feasibility of a plan described as a sequence or a conditional combination of subplans.