Being John Malkovich (1999)

Manhattan, the present. Under pressure from his wife Lotte, street-puppeteer Craig Schwartz takes a job as a clerk with LesterCorp., run by Doctor Lester, a company located on the low-ceilinged seventh-and-a-half floor of an office tower. He falls in love with his co-worker Maxine. Craig discovers a hidden door leading to a passage which sucks him into the head of actor John Malkovich, whose life he experiences for 15 minutes before being ejected on to the side of the New Jersey Turnpike. Craig introduces Lotte to being John Malkovich, and while she is inside him, she/Malkovich makes love to Maxine; the two women fall in love, but can only enjoy each other physically when Lotte is in Malkovich.
Craig and Maxine start a business charging people to be Malkovich. Eventually Malkovich rumbles their scam and enters his portal himself, discovering a world where everyone is him. Craig and Lotte struggle for possession of Maxine. Eventually, Craig uses his puppeteering skills to enter Malkovich permanently and turns Malkovich into a world-famous puppeteer; Maxine becomes his lover. Lester explains to Lotte that Malkovich is the latest in a line of conduits used by a secret society to enjoy eternal life in new bodies. Craig is forced out of Malkovich. Years later, Malkovich has joined the channellers; Lotte and Maxine are a happy couple with a daughter; Craig is inside their daughter, still in love with Maxine, now his mother.