diff --git a/player/alteration.py b/player/alteration.py
new file mode 100644
index 0000000..297f3d9
--- /dev/null
+++ b/player/alteration.py
@@ -0,0 +1,98 @@
+"""The alteration engine: a knob vector -> a layered RenderPlan (design §4, §5).
+
+Replaces the 2026-06-04 nearest-match *selection* with a *transformation* of a
+neutral base clip. Given the four experience knobs, it produces three layers
+that compose per §4.2:
+
+  - Substrate transforms (alter pixels, blend with each other):
+      * Restyle  -- the Right axis: a pre-baked generative v2v dreamlike restyle.
+      * ColorGrade -- the mood axis (Dark/Light): a deterministic color grade.
+  - Overlay (composited on top of the substrate):
+      * AnalyticalOverlay -- the Left axis: HUD/labels/measurement.
+
+Left and Right are NOT opposites; they live on different layers and stack
+(§4.2). Dark and Light are the two poles of one mood grade whose center is the
+identity (§5).
+
+Calibration note: the knob->strength curves below are the single source of
+truth for how a 0..4 position maps to a transform strength. See the session
+0006 transcript Deferred decisions for the §3-vs-§4.2/§5 reconciliation.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from hef.selection import Coordinate
+
+KNOB_MAX = 4  # knob full-scale (0..4)
+
+
+@dataclass(frozen=True)
+class ColorGrade:
+    """Mood-axis grade (§5). `tone` is signed: >0 warm yellow->white (light),
+    <0 cool blue->black (dark), 0 = identity (raw, ungraded)."""
+
+    tone: float
+
+    @property
+    def is_identity(self) -> bool:
+        return self.tone == 0.0
+
+
+@dataclass(frozen=True)
+class AnalyticalOverlay:
+    """Left axis (§4.1): analytical HUD/annotation/labels, composited on top.
+    `intensity` 0..1 (0 = no overlay)."""
+
+    intensity: float
+
+
+@dataclass(frozen=True)
+class Restyle:
+    """Right axis (§4.1): pre-baked generative v2v dreamlike substrate.
+    `blend` 0..1 (0 = raw substrate, no restyle)."""
+
+    blend: float
+
+
+@dataclass(frozen=True)
+class RenderPlan:
+    """The full layered alteration for one knob vector (§4.2)."""
+
+    grade: ColorGrade
+    overlay: AnalyticalOverlay
+    restyle: Restyle
+
+    @property
+    def is_identity(self) -> bool:
+        """True when the plan leaves the neutral base un-altered."""
+        return (
+            self.grade.is_identity
+            and self.overlay.intensity == 0.0
+            and self.restyle.blend == 0.0
+        )
+
+
+def _overlay_intensity(left: int) -> float:
+    """Left knob -> analytical-overlay intensity (0..1)."""
+    return left / KNOB_MAX
+
+
+def _restyle_blend(right: int) -> float:
+    """Right knob -> v2v restyle blend (0..1)."""
+    return right / KNOB_MAX
+
+
+def _mood_tone(dark: int, light: int) -> float:
+    """(dark, light) -> signed mood grade in [-1, 1]; equal -> 0 identity (§5)."""
+    return (light - dark) / KNOB_MAX
+
+
+def plan_alteration(coord: Coordinate) -> RenderPlan:
+    """Map a knob vector to its layered RenderPlan (design §4)."""
+    return RenderPlan(
+        grade=ColorGrade(tone=_mood_tone(coord.dark, coord.light)),
+        overlay=AnalyticalOverlay(intensity=_overlay_intensity(coord.left)),
+        restyle=Restyle(blend=_restyle_blend(coord.right)),
+    )
diff --git a/tests/test_player_alteration.py b/tests/test_player_alteration.py
new file mode 100644
index 0000000..0897f2b
--- /dev/null
+++ b/tests/test_player_alteration.py
@@ -0,0 +1,80 @@
+import pytest
+
+from hef.selection import Coordinate
+from player.alteration import (
+    AnalyticalOverlay,
+    ColorGrade,
+    RenderPlan,
+    Restyle,
+    plan_alteration,
+)
+
+
+def _coord(left=0, right=0, dark=0, light=0):
+    return Coordinate(left=left, right=right, dark=dark, light=light)
+
+
+def test_all_zero_knobs_is_the_unaltered_base():
+    plan = plan_alteration(_coord())
+    assert plan.is_identity
+    assert plan.overlay.intensity == 0.0
+    assert plan.restyle.blend == 0.0
+    assert plan.grade.tone == 0.0
+    assert plan.grade.is_identity
+
+
+def test_left_drives_the_analytical_overlay_only():
+    plan = plan_alteration(_coord(left=4))
+    assert plan.overlay.intensity == 1.0
+    assert plan.restyle.blend == 0.0  # Left does not touch the substrate
+    assert plan.grade.tone == 0.0
+
+
+def test_right_drives_the_restyle_substrate_only():
+    plan = plan_alteration(_coord(right=2))
+    assert plan.restyle.blend == 0.5
+    assert plan.overlay.intensity == 0.0  # Right does not add overlay
+
+
+def test_left_and_right_stack_not_cancel():
+    # design §4.2: whole-brain corner = dreamlike substrate WITH labels on top
+    plan = plan_alteration(_coord(left=4, right=4))
+    assert plan.overlay.intensity == 1.0
+    assert plan.restyle.blend == 1.0
+
+
+def test_light_pole_grades_warm_positive_tone():
+    plan = plan_alteration(_coord(light=4))
+    assert plan.grade.tone == 1.0
+    assert not plan.grade.is_identity
+
+
+def test_dark_pole_grades_cool_negative_tone():
+    plan = plan_alteration(_coord(dark=4))
+    assert plan.grade.tone == -1.0
+
+
+def test_equal_dark_and_light_is_identity_grade():
+    # design §5: the mood center is the raw, ungraded footage
+    assert plan_alteration(_coord(dark=3, light=3)).grade.is_identity
+    assert plan_alteration(_coord(dark=2, light=2)).grade.is_identity
+
+
+def test_dark_minus_light_sets_intermediate_tone():
+    assert plan_alteration(_coord(dark=4, light=2)).grade.tone == pytest.approx(-0.5)
+    assert plan_alteration(_coord(dark=1, light=3)).grade.tone == pytest.approx(0.5)
+
+
+def test_whole_brain_dark_corner_stacks_grade_substrate_and_overlay():
+    # design §4.2 "Dark + analytical": cold measurement over a melancholy scene
+    plan = plan_alteration(_coord(left=4, right=2, dark=4, light=0))
+    assert plan.overlay.intensity == 1.0
+    assert plan.restyle.blend == 0.5
+    assert plan.grade.tone == -1.0
+    assert not plan.is_identity
+
+
+def test_render_plan_is_frozen():
+    plan = plan_alteration(_coord())
+    with pytest.raises(Exception):
+        plan.grade.tone = 0.5  # type: ignore[misc]