The curtain rises, and a lively cocktail party scene unfolds.
Literal
Curtain [subject-が] rises [and-て], lively cocktail-party's scene [into-と] becomes.
幕が上がる ('the curtain rises') is the theatrical set expression — counterpart to 幕が下りる ('the curtain falls'). となる is the formal 'becomes' variant of になる. The construction [scene description]となる is stage-direction language: 'the scene becomes X.'