$\mathbb{R}^2$-tic-tac-toe is a game where two players take turns putting red and blue points anywhere on the $xy$ plane.  The red player moves first.  The first player to get $3$ of their points in a line without any of their opponent's points in between wins.  What is the least number of moves in which Red can guarantee a win?  (We count each time that Red places a point as a move, including when Red places its winning point.)
1. **First Move by Red:**
   - Red places the first point anywhere on the $xy$ plane. This move does not affect the strategy significantly as it is the initial point.

2. **First Move by Blue:**
   - Blue places the first point anywhere on the $xy$ plane. This move also does not affect the strategy significantly as it is the initial point.

3. **Second Move by Red:**
   - Red places the second point. The placement of this point is crucial. Red should place this point such that it forms a potential line with the first point. For example, if the first point is at $(0,0)$, the second point could be at $(1,0)$.

4. **Second Move by Blue:**
   - Blue must place the second point in such a way that it blocks the potential line formed by Red's first two points. For instance, if Red's points are at $(0,0)$ and $(1,0)$, Blue should place a point at $(0.5,0)$ to block the line.

5. **Third Move by Red:**
   - Red places the third point. This point should be placed such that it does not form a degenerate triangle (i.e., all three points should not be collinear). For example, if Red's points are at $(0,0)$ and $(1,0)$, the third point could be at $(0,1)$.

6. **Third Move by Blue:**
   - Blue places the third point. Blue must place this point in such a way that it does not block any potential line formed by Red's points. For instance, if Red's points are at $(0,0)$, $(1,0)$, and $(0,1)$, Blue should place a point that does not interfere with these points forming a line.

7. **Fourth Move by Red:**
   - Red places the fourth point. This point should be placed such that it forms a line with two of the previous points without any of Blue's points in between. For example, if Red's points are at $(0,0)$, $(1,0)$, and $(0,1)$, the fourth point could be at $(1,1)$ to form a line with $(0,0)$ and $(1,1)$.

By following this strategy, Red can guarantee a win in the fourth move.

The final answer is $\boxed{4}$