diff --git a/src/After/Mappers.cs b/src/After/Mappers.cs
index c9f016b..ce9503c 100644
--- a/src/After/Mappers.cs
+++ b/src/After/Mappers.cs
@@ -103,9 +103,11 @@ public static class Mappers
         {
             Id = dto.Id,
             Description = dto.Description,
-            TotalInCents = dto.TotalInCents,
             Client = dto.ClientDto is null ? null : UnmapClient(dto.ClientDto, seen),
         };
+        // Derived data cannot be written, only re-derived: the flat DTO total
+        // becomes exactly one line.
+        order.Lines.Add(new OrderLine(dto.Description, dto.TotalInCents));
         seen[dto] = order;
         return order;
     }
@@ -161,6 +163,10 @@ public static class Mappers
     public static Order FromOrderData(OrderData row)
     {
         ArgumentNullException.ThrowIfNull(row);
-        return new Order { Id = row.Id, Description = row.Description, TotalInCents = row.TotalInCents };
+        var order = new Order { Id = row.Id, Description = row.Description };
+        // Read-back: the flat row becomes exactly one synthetic line, so the
+        // computed total — and everything calculated from it — keeps working.
+        order.Lines.Add(new OrderLine(row.Description, row.TotalInCents));
+        return order;
     }
 }
\ No newline at end of file
diff --git a/src/After/Order.cs b/src/After/Order.cs
index 95777a9..755f7d0 100644
--- a/src/After/Order.cs
+++ b/src/After/Order.cs
@@ -18,11 +18,17 @@ public class Order
     public string Description { get; set; } = string.Empty;
 
     /// <summary>
-    /// Order total in cents (integer — no floating-point money). A normal
-    /// attribute: it is persisted in <see cref="OrderData"/> and mapped
-    /// one-on-one into <see cref="OrderDto.TotalInCents"/>.
+    /// Order total in cents (integer — no floating-point money), computed
+    /// from the order lines. Since the lines arrived this is derived data:
+    /// it cannot be written, only re-derived — which is exactly why the
+    /// inbound mappings (DTO, storage row) reconstruct a one-line order.
+    /// Persisted in <see cref="OrderData"/> and mapped one-on-one into
+    /// <see cref="OrderDto.TotalInCents"/>.
     /// </summary>
-    public int TotalInCents { get; set; }
+    public int TotalInCents => Lines.Sum(line => line.AmountInCents);
+
+    /// <summary>The lines this order consists of. The total is their sum.</summary>
+    public List<OrderLine> Lines { get; } = [];
 
     /// <summary>
     /// The discounted total in cents, copied onto the order by the caller of
diff --git a/src/After/OrderLine.cs b/src/After/OrderLine.cs
new file mode 100644
index 0000000..a7bc3f4
--- /dev/null
+++ b/src/After/OrderLine.cs
@@ -0,0 +1,15 @@
+namespace After;
+
+/// <summary>
+/// One line of an <see cref="Order"/>: its own description and amount. A
+/// plain, mutable domain type in the same style as <see cref="Order"/> and
+/// <see cref="Client"/> (only the DTOs are records).
+/// </summary>
+public class OrderLine(string description, int amountInCents)
+{
+    /// <summary>Free-text description of what this line is for.</summary>
+    public string Description { get; set; } = description;
+
+    /// <summary>Line amount in cents (integer — no floating-point money).</summary>
+    public int AmountInCents { get; set; } = amountInCents;
+}
diff --git a/tests/BeforeAfter.Tests/AfterPersistenceTests.cs b/tests/BeforeAfter.Tests/AfterPersistenceTests.cs
index 08db56b..6981b9a 100644
--- a/tests/BeforeAfter.Tests/AfterPersistenceTests.cs
+++ b/tests/BeforeAfter.Tests/AfterPersistenceTests.cs
@@ -55,7 +55,7 @@ public class AfterPersistenceTests
         db.Save(order);
 
         order.Description = "one (edited)";
-        order.TotalInCents = 99_00;
+        order.Lines[0].AmountInCents = 99_00;
         client.Name = "Acme GmbH";
         db.Save(order);
         db.Save(client);
diff --git a/tests/BeforeAfter.Tests/OrderLinesEvolutionTests.cs b/tests/BeforeAfter.Tests/OrderLinesEvolutionTests.cs
new file mode 100644
index 0000000..e9177ce
--- /dev/null
+++ b/tests/BeforeAfter.Tests/OrderLinesEvolutionTests.cs
@@ -0,0 +1,51 @@
+using After;
+using static BeforeAfter.Tests.TestData.ClientBuilder;
+using static BeforeAfter.Tests.TestData.OrderBuilder;
+
+namespace BeforeAfter.Tests;
+
+/// <summary>
+/// EXERCISE 2 — the domain gained OrderLines; persistence still stores one
+/// flat row per order. (This is the solution participants arrive at from the
+/// stub in EXERCISE2.md.)
+/// </summary>
+public class OrderLinesEvolutionTests
+{
+    [Fact] // EXERCISE STEP 3a
+    public void A_reloaded_order_can_be_discounted()
+    {
+        var db = new DbContext();
+        var order = ValidOrder().WithDescription("big order").WithTotal(150_00).Build();
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
+        db.Save(client);
+        db.Save(order);
+
+        var loaded = db.LoadOrder(order.Id)!;
+
+        // The discount works off the computed total of the RELOADED order:
+        var discounted = DiscountCalculator.CalculateDiscount(
+            loaded, discountBarrierCents: 100_00, discountPercentage: 10);
+
+        Assert.Equal(135_00, discounted.DiscountedTotalInCents);
+        Assert.Equal(10, discounted.DiscountPercentage);
+    }
+
+    [Fact] // EXERCISE STEP 3b — the accepted loss
+    public void A_multi_line_order_flattens_to_a_single_line_when_stored()
+    {
+        var db = new DbContext();
+        var order = ValidOrder().WithDescription("mixed order").Build();
+        order.Lines.Add(new OrderLine("first item", 100_00));
+        order.Lines.Add(new OrderLine("second item", 50_00));
+
+        db.Save(order);
+
+        var loaded = db.LoadOrder(order.Id)!;
+
+        // Storage flattened the order: one line, the flat total, and the
+        // order-level description. Documented, not hidden.
+        var line = Assert.Single(loaded.Lines);
+        Assert.Equal(150_00, line.AmountInCents);
+        Assert.Equal("mixed order", line.Description);
+    }
+}
diff --git a/tests/BeforeAfter.Tests/TestData/OrderBuilder.cs b/tests/BeforeAfter.Tests/TestData/OrderBuilder.cs
index d9a33a6..5099547 100644
--- a/tests/BeforeAfter.Tests/TestData/OrderBuilder.cs
+++ b/tests/BeforeAfter.Tests/TestData/OrderBuilder.cs
@@ -16,6 +16,7 @@ namespace BeforeAfter.Tests.TestData;
 public sealed class OrderBuilder
 {
     private readonly Order _order = new();
+    private int? _totalInCents;
 
     /// <summary>A valid, fresh order — the neutral starting point for tests.</summary>
     public static OrderBuilder ValidOrder() => new();
@@ -33,15 +34,23 @@ public sealed class OrderBuilder
     }
 
     /// <summary>
-    /// Sets the total in cents (<c>150_00</c> = €150.00). How the requested
-    /// total is realised inside the order is an implementation detail that may
-    /// change as the domain evolves; this API stays.
+    /// Sets the total in cents (<c>150_00</c> = €150.00). Since OrderLines
+    /// arrived, the total is computed — the requested total is realised in
+    /// <see cref="Build"/> as a single line carrying the order's description,
+    /// so <c>WithDescription</c>/<c>WithTotal</c> work in any order. This API
+    /// stayed identical across that domain evolution; only these internals
+    /// changed.
     /// </summary>
     public OrderBuilder WithTotal(int totalInCents)
     {
-        _order.TotalInCents = totalInCents;
+        _totalInCents = totalInCents;
         return this;
     }
 
-    public Order Build() => _order;
+    public Order Build()
+    {
+        if (_totalInCents is int total)
+            _order.Lines.Add(new OrderLine(_order.Description, total));
+        return _order;
+    }
 }