diff --git a/src/AcDream.Core/Physics/PortalPlane.cs b/src/AcDream.Core/Physics/PortalPlane.cs
new file mode 100644
index 0000000..f81611b
--- /dev/null
+++ b/src/AcDream.Core/Physics/PortalPlane.cs
@@ -0,0 +1,45 @@
+using System.Numerics;
+
+namespace AcDream.Core.Physics;
+
+/// <summary>
+/// A portal plane derived from an EnvCell's CellPortal polygon.
+/// Used to detect when a player crosses from one cell into another.
+/// </summary>
+public readonly record struct PortalPlane(
+    Vector3 Normal,
+    float D,
+    uint TargetCellId,   // OtherCellId — the cell on the far side (0xFFFF = outdoor)
+    uint OwnerCellId,    // the EnvCell that owns this portal
+    ushort Flags)        // PortalFlags value
+{
+    /// <summary>
+    /// Construct a PortalPlane from three coplanar vertices (winding order
+    /// determines the normal direction via cross product).
+    /// </summary>
+    public static PortalPlane FromVertices(
+        Vector3 v0, Vector3 v1, Vector3 v2,
+        uint targetCellId, uint ownerCellId, ushort flags)
+    {
+        var edge1 = v1 - v0;
+        var edge2 = v2 - v0;
+        var normal = Vector3.Normalize(Vector3.Cross(edge1, edge2));
+        float d = -Vector3.Dot(normal, v0);
+        return new PortalPlane(normal, d, targetCellId, ownerCellId, flags);
+    }
+
+    /// <summary>
+    /// Returns true when the movement from <paramref name="oldPos"/> to
+    /// <paramref name="newPos"/> crosses this plane (the two positions are on
+    /// strictly opposite sides).  A position exactly on the plane (distance = 0)
+    /// does NOT count as a crossing.
+    /// </summary>
+    public bool IsCrossing(Vector3 oldPos, Vector3 newPos)
+    {
+        float oldDist = Vector3.Dot(Normal, oldPos) + D;
+        float newDist = Vector3.Dot(Normal, newPos) + D;
+        // Strictly negative product → opposite signs → crossed the plane.
+        // If either distance is exactly 0 the product is 0, not negative → no crossing.
+        return oldDist * newDist < 0f;
+    }
+}
diff --git a/tests/AcDream.Core.Tests/Physics/PortalPlaneTests.cs b/tests/AcDream.Core.Tests/Physics/PortalPlaneTests.cs
new file mode 100644
index 0000000..70ef9d1
--- /dev/null
+++ b/tests/AcDream.Core.Tests/Physics/PortalPlaneTests.cs
@@ -0,0 +1,67 @@
+using System.Numerics;
+using AcDream.Core.Physics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+public class PortalPlaneTests
+{
+    // Helper: build an XY-plane portal (z = 0) from three known vertices.
+    private static PortalPlane XyPlane() =>
+        PortalPlane.FromVertices(
+            new Vector3(0, 0, 0),
+            new Vector3(1, 0, 0),
+            new Vector3(0, 1, 0),
+            targetCellId: 42,
+            ownerCellId: 7,
+            flags: 0);
+
+    [Fact]
+    public void FromVertices_ComputesCorrectNormal()
+    {
+        var plane = XyPlane();
+
+        // Cross((1,0,0)-(0,0,0), (0,1,0)-(0,0,0)) = Cross((1,0,0),(0,1,0)) = (0,0,1)
+        // The normalised result must be either (0,0,1) or (0,0,-1).
+        Assert.Equal(0f, plane.Normal.X, precision: 5);
+        Assert.Equal(0f, plane.Normal.Y, precision: 5);
+        Assert.Equal(1f, MathF.Abs(plane.Normal.Z), precision: 5);
+    }
+
+    [Fact]
+    public void IsCrossing_PositionsOnOppositeSides_ReturnsTrue()
+    {
+        var plane = XyPlane(); // normal is (0,0,±1), D = 0
+
+        // One position above the XY plane, one below.
+        var above = new Vector3(0, 0, 1f);
+        var below = new Vector3(0, 0, -1f);
+
+        Assert.True(plane.IsCrossing(above, below));
+        Assert.True(plane.IsCrossing(below, above));
+    }
+
+    [Fact]
+    public void IsCrossing_PositionsOnSameSide_ReturnsFalse()
+    {
+        var plane = XyPlane();
+
+        var pos1 = new Vector3(0, 0, 1f);
+        var pos2 = new Vector3(0, 0, 2f);
+
+        Assert.False(plane.IsCrossing(pos1, pos2));
+    }
+
+    [Fact]
+    public void IsCrossing_StartOnPlane_ReturnsFalse()
+    {
+        var plane = XyPlane();
+
+        // oldPos is exactly on the plane (z = 0) → distance = 0.
+        // Product (0 * anything) == 0, which is NOT < 0 → no crossing.
+        var onPlane = new Vector3(0.5f, 0.5f, 0f);
+        var above   = new Vector3(0.5f, 0.5f, 1f);
+
+        Assert.False(plane.IsCrossing(onPlane, above));
+    }
+}