diff --git a/tests/acceptance/Acceptance.csproj b/tests/acceptance/Acceptance.csproj
index a1f3232..c182441 100644
--- a/tests/acceptance/Acceptance.csproj
+++ b/tests/acceptance/Acceptance.csproj
@@ -11,6 +11,7 @@
     <PackageReference Include="coverlet.collector" Version="6.0.4" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
     <PackageReference Include="Reqnroll.xUnit" Version="3.3.4" />
+    <PackageReference Include="xunit" Version="2.9.3" />
     <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
   </ItemGroup>
 
diff --git a/tests/acceptance/Steps/EenZaakOpenenSteps.cs b/tests/acceptance/Steps/EenZaakOpenenSteps.cs
new file mode 100644
index 0000000..8c191bc
--- /dev/null
+++ b/tests/acceptance/Steps/EenZaakOpenenSteps.cs
@@ -0,0 +1,65 @@
+using Acceptance.Support;
+using Acl.Application;
+using Reqnroll;
+using Xunit;
+
+namespace Acceptance.Steps;
+
+/// <summary>Bindings for <c>EenZaakOpenen.feature</c>. Reqnroll creates one
+/// instance per scenario, so instance fields hold scenario-scoped state.</summary>
+[Binding]
+public sealed class EenZaakOpenenSteps
+{
+    private readonly InMemoryZaakGateway _gateway = new();
+    private DomainRegistration? _registration;
+    private AclDefaults? _defaults;
+    private DateOnly _today;
+    private Uri? _returnedUrl;
+
+    [Given("a domain registration for BSN \"(.*)\"")]
+    public void GivenADomainRegistrationForBsn(string bsn)
+        => _registration = new DomainRegistration(bsn);
+
+    [Given("the ACL is configured with these defaults:")]
+    public void GivenTheAclIsConfiguredWithTheseDefaults(DataTable defaults)
+    {
+        var values = ToFieldMap(defaults);
+        _defaults = new AclDefaults
+        {
+            Bronorganisatie = values["bronorganisatie"],
+            VerantwoordelijkeOrganisatie = values["verantwoordelijkeOrganisatie"],
+            Vertrouwelijkheidaanduiding = values["vertrouwelijkheidaanduiding"],
+            ZaaktypeUrl = new Uri(values["zaaktype"]),
+        };
+    }
+
+    [Given("today is \"(.*)\"")]
+    public void GivenTodayIs(string date)
+        => _today = DateOnly.Parse(date);
+
+    [When("the domain asks the ACL to open a zaak")]
+    public async Task WhenTheDomainAsksTheAclToOpenAZaak()
+    {
+        var service = new AclService(_gateway, _defaults!, new FixedClock(_today));
+        _returnedUrl = await service.OpenZaakAsync(_registration!);
+    }
+
+    [Then("a zaak is created with these default-filled fields")]
+    public void ThenAZaakIsCreatedWithTheseDefaultFilledFields(DataTable expected)
+    {
+        var request = Assert.IsType<ZaakRequest>(_gateway.Captured);
+        var fields = ToFieldMap(expected);
+        Assert.Equal(fields["bronorganisatie"], request.Bronorganisatie);
+        Assert.Equal(fields["verantwoordelijkeOrganisatie"], request.VerantwoordelijkeOrganisatie);
+        Assert.Equal(fields["vertrouwelijkheidaanduiding"], request.Vertrouwelijkheidaanduiding);
+        Assert.Equal(fields["zaaktype"], request.Zaaktype.ToString());
+        Assert.Equal(fields["startdatum"], request.Startdatum.ToString("yyyy-MM-dd"));
+    }
+
+    [Then("the ACL returns the URL of the created zaak")]
+    public void ThenTheAclReturnsTheUrlOfTheCreatedZaak()
+        => Assert.Equal(InMemoryZaakGateway.CreatedZaakUrl, _returnedUrl);
+
+    private static Dictionary<string, string> ToFieldMap(DataTable table)
+        => table.Rows.ToDictionary(r => r["field"], r => r["value"]);
+}
diff --git a/tests/acceptance/Support/FixedClock.cs b/tests/acceptance/Support/FixedClock.cs
new file mode 100644
index 0000000..b6d8dce
--- /dev/null
+++ b/tests/acceptance/Support/FixedClock.cs
@@ -0,0 +1,10 @@
+using Acl.Application;
+
+namespace Acceptance.Support;
+
+/// <summary>A clock pinned to a known date so <c>startdatum</c> is deterministic
+/// in scenarios (ADR-0003).</summary>
+public sealed class FixedClock(DateOnly today) : IClock
+{
+    public DateOnly Today { get; } = today;
+}
diff --git a/tests/acceptance/Support/InMemoryZaakGateway.cs b/tests/acceptance/Support/InMemoryZaakGateway.cs
new file mode 100644
index 0000000..a1dde7b
--- /dev/null
+++ b/tests/acceptance/Support/InMemoryZaakGateway.cs
@@ -0,0 +1,20 @@
+using Acl.Application;
+
+namespace Acceptance.Support;
+
+/// <summary>An in-memory stand-in for the OpenZaak Zaken API. It captures the
+/// fully default-filled <see cref="ZaakRequest"/> the ACL builds and returns a
+/// fixed zaak URL, so the acceptance scenario can verify the use case without a
+/// running OpenZaak (real-OpenZaak verification is a separate slice).</summary>
+public sealed class InMemoryZaakGateway : IZaakGateway
+{
+    public static readonly Uri CreatedZaakUrl = new("http://openzaak/zaken/api/v1/zaken/created-123");
+
+    public ZaakRequest? Captured { get; private set; }
+
+    public Task<Uri> OpenZaakAsync(ZaakRequest request, CancellationToken ct = default)
+    {
+        Captured = request;
+        return Task.FromResult(CreatedZaakUrl);
+    }
+}