diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0da82a25..6d971cf3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -3,6 +3,7 @@ cmake_minimum_required(VERSION 3.13 FATAL_ERROR)
 project(isle CXX)
 
 option(ISLE_BUILD_APP "Build ISLE.EXE application" ON)
+option(ISLE_BUILD_PATCH "Build patching into ISLE.EXE and LEGO1.DLL (requires ISLE_BUILD_APP)" OFF)
 option(ISLE_USE_SMARTHEAP "Build with SmartHeap" ${MSVC})
 option(ISLE_USE_DX5 "Build with internal DirectX 5 SDK" ON)
 
@@ -219,6 +220,22 @@ target_link_libraries(lego1 PRIVATE ddraw dsound dxguid dinput winmm)
 set_property(TARGET lego1 PROPERTY OUTPUT_NAME LEGO1)
 set_property(TARGET lego1 PROPERTY SUFFIX ".DLL")
 
+# Forward ISLE_BUILD_PATCH
+if (ISLE_BUILD_PATCH)
+  target_sources(lego1 PRIVATE LEGO1/decomp.cpp)
+
+  target_compile_definitions(lego1 PRIVATE ISLE_BUILD_PATCH)
+
+  # Copy LEGO1.DLL to LEGO1_PATCH.DLL after LEGO1.DLL is built
+  # This is a crappy hack, but when building the patcher, we want
+  # ISLE.EXE to import the original LEGO1.DLL, then load LEGO1_PATCH.DLL
+  # (our DLL) manually
+  add_custom_command(
+    TARGET lego1 POST_BUILD
+    COMMAND ${CMAKE_COMMAND} -E copy_if_different $<TARGET_FILE:lego1> $<TARGET_FILE_DIR:lego1>/LEGO1_PATCH.DLL
+  )
+endif()
+
 if (ISLE_BUILD_APP)
   add_executable(isle WIN32
     ISLE/res/isle.rc
@@ -245,6 +262,11 @@ if (ISLE_BUILD_APP)
   # Make sure filenames are ALL CAPS
   set_property(TARGET isle PROPERTY OUTPUT_NAME ISLE)
   set_property(TARGET isle PROPERTY SUFFIX ".EXE")
+
+  # Forward ISLE_BUILD_PATCH
+  if (ISLE_BUILD_PATCH)
+    target_compile_definitions(isle PRIVATE ISLE_BUILD_PATCH)
+  endif()
 endif()
 
 if (MSVC)
@@ -272,4 +294,4 @@ if (MSVC)
   set(CMAKE_SHARED_LINKER_FLAGS_RELEASE "/incremental:no")
   set(CMAKE_SHARED_LINKER_FLAGS_RELWITHDEBINFO "/incremental:no /debug")
   set(CMAKE_SHARED_LINKER_FLAGS_MINSIZEREL "/incremental:no")
-endif()
\ No newline at end of file
+endif()
diff --git a/ISLE/isleapp.cpp b/ISLE/isleapp.cpp
index c20f1652..bc426e49 100644
--- a/ISLE/isleapp.cpp
+++ b/ISLE/isleapp.cpp
@@ -155,6 +155,21 @@ BOOL StartDirectSound(void);
 // OFFSET: ISLE 0x401610
 int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd)
 {
+#ifdef ISLE_BUILD_PATCH
+  // Load LEGO1_PATCH.DLL
+  {
+    HMODULE hModule = LoadLibraryA("LEGO1_PATCH.DLL");
+    if (hModule) {
+      typedef void (*PatchFunc)(void*);
+      PatchFunc patchFunc = (PatchFunc)GetProcAddress(hModule, "Patch");
+      if (patchFunc) {
+        void *root = (char*)VideoManager - 0x10015720;
+        patchFunc(root);
+      }
+    }
+  }
+#endif
+
   // Look for another instance, if we find one, bring it to the foreground instead
   if (!FindExistingInstance()) {
     return 0;
diff --git a/LEGO1/decomp.cpp b/LEGO1/decomp.cpp
new file mode 100644
index 00000000..dee2a0a2
--- /dev/null
+++ b/LEGO1/decomp.cpp
@@ -0,0 +1,9 @@
+#include "decomp.h"
+
+#include "Windows.h"
+
+// Export "Patch" function (non-mangled name)
+extern "C" __declspec(dllexport) void Patch(void *root)
+{
+  MessageBoxA(NULL, "HELLO", "HELLO", 0);
+}
diff --git a/LEGO1/decomp.h b/LEGO1/decomp.h
index cd179ae6..f47a2343 100644
--- a/LEGO1/decomp.h
+++ b/LEGO1/decomp.h
@@ -12,4 +12,21 @@ typedef unsigned char undefined;
 typedef unsigned short undefined2;
 typedef unsigned int undefined4;
 
+#ifdef ISLE_BUILD_PATCH
+
+class PatchHook
+{
+public:
+  PatchHook(void *p_ourFunc, void *p_origFunc);
+};
+
+#define PATCH_HOOK(ourFunc, origFunc) \
+  static PatchHook _patchHook_##__COUNTER__ ((void *)ourFunc, (void *)origFunc)
+
+#else
+
+#define PATCH_HOOK(ourFunc, origFunc)
+
+#endif
+
 #endif // DECOMP_H
diff --git a/LEGO1/mxtransitionmanager.cpp b/LEGO1/mxtransitionmanager.cpp
index f66f4bae..c985aaec 100644
--- a/LEGO1/mxtransitionmanager.cpp
+++ b/LEGO1/mxtransitionmanager.cpp
@@ -192,6 +192,11 @@ MxResult MxTransitionManager::StartTransition(TransitionType p_animationType, Mx
   return FAILURE;
 }
 
+void Test()
+{
+  MxResult (MxTransitionManager:: * pfTarget)(MxTransitionManager::TransitionType, MxS32, MxBool, MxBool) = MxTransitionManager::StartTransition;
+}
+
 // OFFSET: LEGO1 0x1004c170
 void MxTransitionManager::Transition_Wipe()
 {