As jberryman has expressed, the real issue here is what is meant by a "line" and a "plane". Traditionally, these are taken as undefined primitive concepts, and the ideas that any two points will determine a unique line, and that if two points are in a plane, then the line through those two points will lie entirely in that plane are taken as axioms.
The problem with definitions is that they can only introduce new ideas in terms of older ideas. But you have to start somewhere. The base terminology of a theory, such as geometry, cannot be defined in a normal fashion. Instead, we call these terms "primitives", and choose certain statements, called "axioms" or "postulates" relating them to each other which we simply assert to be true. It is this list of primitives and axioms that establish what the theory we are working on is. In a sense, the axioms define the primitives, by establishing how they relate to each other.
While it is common to treat "line" and "plane" as primitives, it is not absolutely necessary. You can build up all of Euclidean geometry just from the primitive term "point" and the primitive relationship "these two points are closer together than those two points", and basic set theory. (It is not wise to do so, as it requires a lot of niggling axioms to make sure that these primitives encompass the desired behavior, which would thoroughly confuse new students of the subject.)
The general outline of this approach is:
- Define distance by equivalence classes on the relation "$A$ and $B$ are neither closer together nor farther apart than $C$ and $D$"
- Define "$B$ is between $A$ and $C$" if among all points $D$ the same distance from $B$ as $C$, $C$ is the farthest from $A$.
- Define "$A,B,C$ are colinear" if one is between the other two.
- Define a set of points to be "linearly closed" if for any two points $A, B$ in the set, if $C$ is colinear with them, then $C$ is also in the set.
- A "line" is a linearly closed set where every trio of points are colinear.
- The "span" of a set of points is the smallest linearly closed set of points containing it.
- A "plane" is the span of some set of three non-colinear points.
With this system, it is by definition that once a line and a plane share two points, the line must lie in the plane, as every point on the line must be colinear with those two points, and therefore must be in the plane, as the plane is linearly closed.
Whether you choose to do some grandiose scheme of definitions as above, or take the simple route of having "line" and "plane" as primitives and make this condition an axiom, it is something you want to have in your geometry, because it expresses the idea of what we want a plane to be: something that expands the idea of "straightness" into another direction.