It all starts when the two leading actor and actress meet each other
"It all starts"=「全てはここから始まる」
"when the two leading actor and actress meet"=「二人の主人公(男)と主人公(女)が会うとき」
"each other"=「お互いと」
"the male lead"と"the female lead"という単語も使えますが、"the leading actor"、"the leading actress"とも言えます。