diff --git a/app/controllers/stages_controller.rb b/app/controllers/stages_controller.rb
new file mode 100644
index 0000000..cdc5f1b
--- /dev/null
+++ b/app/controllers/stages_controller.rb
@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+class StagesController < ApplicationController
+  before_action :set_stage, only: %i[show update]
+  before_action :authenticate_user!, only: %i[update]
+  before_action -> { require_owner! @stage.owner }, only: %i[update]
+
+  # GET /stages/1
+  def show
+    render json: @stage, include: '**'
+  end
+
+  # PUT /stages/1
+  def update
+    if stage_params[:state] == 'finished'
+      unless @stage.state == 'in_progress'
+        render json: { error: 'Only running group stages can be finished' }, status: :unprocessable_entity
+        return
+      end
+
+      Stage.transaction do
+        if @stage.update(stage_params)
+          handle_group_stage_end
+
+          render json: @stage
+        else
+          render json: @stage.errors, status: :unprocessable_entity
+          raise ActiveRecord::Rollback
+        end
+      end
+    else
+      render json: {
+        error: 'The state attribute may only be changed to finished'
+      }, status: :unprocessable_entity
+    end
+  end
+
+  private
+
+  def handle_group_stage_end
+    unless @stage.over?
+      render json: {
+        error: 'Group Stage still has some matches that are not over yet. Finish them to generate playoffs'
+      }, status: :unprocessable_entity
+      raise ActiveRecord::Rollback
+    end
+
+    return if AddPlayoffsToTournamentAndSave.call(tournament: @stage.tournament,
+                                                  teams: GroupStageService.get_advancing_teams(@stage)).success?
+
+    render json: { error: 'Generating group stage failed' }, status: :unprocessable_entity
+    raise ActiveRecord::Rollback
+  end
+
+  def set_stage
+    @stage = Stage.find(params[:id])
+  end
+
+  def stage_params
+    params.slice(:state).permit!
+  end
+end
diff --git a/config/routes.rb b/config/routes.rb
index d8f761a..61901bd 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -9,6 +9,7 @@ Rails.application.routes.draw do
   resources :matches, only: %i[show update] do
     resources :bets, only: %i[index create]
   end
+  resources :stages, only: %i[show update]
   resources :teams, only: %i[show update]
   resources :tournaments do
     resources :statistics, only: %i[index]
diff --git a/spec/controllers/stages_controller_spec.rb b/spec/controllers/stages_controller_spec.rb
new file mode 100644
index 0000000..042db52
--- /dev/null
+++ b/spec/controllers/stages_controller_spec.rb
@@ -0,0 +1,77 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe StagesController, type: :controller do
+  before do
+    @stage = create(:playoff_stage)
+  end
+
+  describe 'GET #show' do
+    it 'returns a success response' do
+      get :show, params: { id: @stage.to_param }
+      expect(response).to be_successful
+    end
+
+    it 'should return the correct stage' do
+      get :show, params: { id: @stage.to_param }
+      body = deserialize_response response
+      expect(Stage.find_by(id: body[:id])).to eq(@stage)
+      expect(body[:level]).to eq(@stage.level)
+      expect(body[:state]).to eq(@stage.state)
+    end
+  end
+
+  describe 'PUT #update' do
+    context 'group_stage with matches that are done' do
+      before do
+        @running_group_stage = create(:group_stage, match_factory: :finished_group_match)
+      end
+
+      FINISHED = { state: 'finished' }.freeze
+
+      it 'doesn\'t have any other stages besides it before update' do
+        expect(@running_group_stage.tournament.stages.size).to eq(1)
+      end
+
+      context 'as owner' do
+        before(:each) do
+          apply_authentication_headers_for @running_group_stage.owner
+        end
+
+        before do
+          put :update, params: { id: @running_group_stage.to_param }.merge(FINISHED)
+          @running_group_stage.reload
+        end
+
+        it 'succeeds' do
+          expect(response).to be_successful
+        end
+
+        it 'stops the stage' do
+          expect(@running_group_stage.state).to eq(FINISHED[:state])
+        end
+
+        it 'adds new stages to the tournament' do
+          expect(@running_group_stage.tournament.stages.size).to be > 1
+        end
+
+        it 'adds the right teams' do
+          expect(@running_group_stage.tournament.stages.max_by(&:level).teams)
+            .to match_array(GroupStageService.get_advancing_teams(@running_group_stage))
+        end
+      end
+
+      context 'as another user' do
+        before(:each) do
+          apply_authentication_headers_for create(:user)
+        end
+
+        it 'returns an error' do
+          put :update, params: { id: @stage.to_param }.merge(FINISHED)
+          expect(response).to have_http_status(:forbidden)
+        end
+      end
+    end
+  end
+end