diff --git a/src/__tests__/plan.test.ts b/src/__tests__/plan.test.ts
new file mode 100644
index 0000000..da150e0
--- /dev/null
+++ b/src/__tests__/plan.test.ts
@@ -0,0 +1,121 @@
+import { describe, it, expect, beforeAll, afterAll, beforeEach } from "vitest";
+import prisma from "../config/database";
+import { resolveCell } from "../services/plan.service";
+
+// ---------------------------------------------------------------------------
+// Test fixtures
+// ---------------------------------------------------------------------------
+
+/** Note-prefix for test-created rows so we can clean them up safely. */
+const N = "wh_plan_";
+
+let adminUserId: number;
+
+beforeAll(async () => {
+  // Pick the first admin user from the test database as our fixture.
+  // The users -> roles relation is named `roles` in the Prisma schema
+  // (users.role_id references roles.id).
+  const admin = await prisma.users.findFirst({
+    where: { roles: { name: "admin" } },
+  });
+  if (!admin) throw new Error("Test setup: admin user not found");
+  adminUserId = admin.id;
+});
+
+beforeEach(async () => {
+  // Clean up any leftover test data from previous runs. We only touch rows
+  // whose `note` contains our test prefix, so we never disturb real data.
+  await prisma.work_plan_entries.deleteMany({
+    where: { note: { contains: N } },
+  });
+  await prisma.work_plan_overrides.deleteMany({
+    where: { note: { contains: N } },
+  });
+});
+
+afterAll(async () => {
+  // Final cleanup so we never leave soft-deleted test rows behind.
+  await prisma.work_plan_entries.deleteMany({
+    where: { note: { contains: N } },
+  });
+  await prisma.work_plan_overrides.deleteMany({
+    where: { note: { contains: N } },
+  });
+});
+
+// ---------------------------------------------------------------------------
+// resolveCell
+// ---------------------------------------------------------------------------
+
+describe("plan.service.resolveCell", () => {
+  it("returns null when nothing covers the date", async () => {
+    // No entry, no override for (adminUserId, "2099-01-01")
+    const result = await resolveCell(adminUserId, "2099-01-01");
+    expect(result).toBeNull();
+  });
+
+  it("returns the entry that covers the date", async () => {
+    await prisma.work_plan_entries.create({
+      data: {
+        user_id: adminUserId,
+        date_from: new Date("2099-06-01"),
+        date_to: new Date("2099-06-10"),
+        category: "work",
+        note: `${N}PLC upgrade`,
+        created_by: adminUserId,
+      },
+    });
+    const result = await resolveCell(adminUserId, "2099-06-05");
+    expect(result).not.toBeNull();
+    expect(result?.source).toBe("entry");
+    expect(result?.note).toBe(`${N}PLC upgrade`);
+    expect(result?.category).toBe("work");
+    expect(result?.rangeFrom).toBe("2099-06-01");
+    expect(result?.rangeTo).toBe("2099-06-10");
+  });
+
+  it("returns the override for that day, not the entry", async () => {
+    // An entry covers 2099-06-01..2099-06-10, but an override on 2099-06-05
+    // takes precedence.
+    await prisma.work_plan_entries.create({
+      data: {
+        user_id: adminUserId,
+        date_from: new Date("2099-06-01"),
+        date_to: new Date("2099-06-10"),
+        category: "work",
+        note: `${N}PLC upgrade`,
+        created_by: adminUserId,
+      },
+    });
+    await prisma.work_plan_overrides.create({
+      data: {
+        user_id: adminUserId,
+        shift_date: new Date("2099-06-05"),
+        category: "leave",
+        note: `${N}Volno po noční`,
+        created_by: adminUserId,
+      },
+    });
+    const result = await resolveCell(adminUserId, "2099-06-05");
+    expect(result).not.toBeNull();
+    expect(result?.source).toBe("override");
+    expect(result?.note).toBe(`${N}Volno po noční`);
+    expect(result?.category).toBe("leave");
+  });
+
+  it("ignores soft-deleted entries and overrides", async () => {
+    await prisma.work_plan_entries.create({
+      data: {
+        user_id: adminUserId,
+        date_from: new Date("2099-06-01"),
+        date_to: new Date("2099-06-10"),
+        category: "work",
+        note: `${N}deleted entry`,
+        is_deleted: true,
+        created_by: adminUserId,
+      },
+    });
+    const result = await resolveCell(adminUserId, "2099-06-05");
+    expect(result).toBeNull();
+  });
+});
diff --git a/src/services/plan.service.ts b/src/services/plan.service.ts
new file mode 100644
index 0000000..5d5f707
--- /dev/null
+++ b/src/services/plan.service.ts
@@ -0,0 +1,98 @@
+import prisma from "../config/database";
+
+/**
+ * Resolved plan cell for a single (user, date). Either from a range entry
+ * (work_plan_entries) or from a single-day override (work_plan_overrides).
+ */
+export interface ResolvedCell {
+  /** Where the cell came from. */
+  source: "entry" | "override";
+  /** ID of the work_plan_entries row, or null when source is "override". */
+  entryId: number | null;
+  /** ID of the work_plan_overrides row, or null when source is "entry". */
+  overrideId: number | null;
+  user_id: number;
+  /** The date the cell resolves to (echoes the input, YYYY-MM-DD). */
+  shift_date: string;
+  project_id: number | null;
+  category: string;
+  note: string;
+  /** Original entry range, or null when source is "override". */
+  rangeFrom: string | null;
+  rangeTo: string | null;
+}
+
+/**
+ * Compute the effective plan cell for (userId, dateStr).
+ *
+ * Precedence: override > entry > null.
+ * Soft-deleted rows are ignored (is_deleted = false filter).
+ * If multiple entries cover the same day, the latest by created_at wins
+ * and a warning is logged.
+ *
+ * dateStr must be a YYYY-MM-DD string. The function builds a JS Date
+ * from it and Prisma handles timezone conversion against the MySQL
+ * @db.Date columns.
+ */
+export async function resolveCell(
+  userId: number,
+  dateStr: string,
+): Promise<ResolvedCell | null> {
+  const date = new Date(dateStr);
+
+  // 1. Single-day override for this exact date
+  const override = await prisma.work_plan_overrides.findFirst({
+    where: { user_id: userId, shift_date: date, is_deleted: false },
+  });
+  if (override) {
+    return {
+      source: "override",
+      entryId: null,
+      overrideId: override.id,
+      user_id: override.user_id,
+      shift_date: dateStr,
+      project_id: override.project_id,
+      category: override.category,
+      note: override.note,
+      rangeFrom: null,
+      rangeTo: null,
+    };
+  }
+
+  // 2. Range entry that covers the day
+  const entries = await prisma.work_plan_entries.findMany({
+    where: {
+      user_id: userId,
+      date_from: { lte: date },
+      date_to: { gte: date },
+      is_deleted: false,
+    },
+    orderBy: { created_at: "desc" },
+  });
+
+  if (entries.length === 0) return null;
+
+  if (entries.length > 1) {
+    // Multiple entries overlap. Use the latest. A persistent audit-log
+    // entry would be ideal here, but logAudit() currently requires a
+    // FastifyRequest — services have no request. Use console.warn as a
+    // stand-in until a service-context audit logger is introduced.
+    console.warn(
+      `[plan.service] multiple entries cover user ${userId} on ${dateStr}; using the latest (entry id ${entries[0].id})`,
+    );
+  }
+
+  const entry = entries[0];
+  return {
+    source: "entry",
+    entryId: entry.id,
+    overrideId: null,
+    user_id: entry.user_id,
+    shift_date: dateStr,
+    project_id: entry.project_id,
+    category: entry.category,
+    note: entry.note,
+    rangeFrom: entry.date_from.toISOString().slice(0, 10),
+    rangeTo: entry.date_to.toISOString().slice(0, 10),
+  };
+}