The Triangle is episode 14 of Season 5 of Growing Pains.
Summary[]
During rehearsals for a play, Mike plants a passionate kiss on the lips of leading lady Kate. Instantly he forgets that he's merely a character in a fictional charade, and falls madly in love with the girl for real. There's just one hitch: Kate is the girlfriend of Mike's pal David, who also happens to be the play's director!
Cast[]