“This is a case about a broken promise, a family disagreement, and an art masterpiece that, if this Court does not step in now to save it, will be lost to the people who love it, and to New York, forever.”

This is not the opening voice-over for a Netflix drama. It is, rather, the first sentence in a lawsuit filed on Thursday in State Supreme Court in Manhattan against the auction giant Sotheby’s by Hubert Neumann, a New York art collector whose family famously owns, and closely guards, one of the most staggering private collections of 20th-century art in the United States.

The art masterpiece in question — “Flesh and Spirit,” a 12-by-12-foot painting by Jean-Michel Basquiat — is just days away from the auction block. It is a prized asset of the estate of Mr. Neumann’s wife, Dolores Ormandy Neumann, who died in September 2016, and its potential sale shines a spotlight on what appears to be a nasty family dispute.

Shortly before her death, Ms. Neumann executed a will that fully disinherited her husband of 62 years and gave their middle child, Belinda, the vast majority of Ms. Neumann’s property while appointing her the preliminary executor of her mother’s estate, according to Mr. Neumann’s lawsuit. The couple’s other daughters, Melissa and Kristina, were left with only modest shares, the suit states.